ABSTRACT THE EFFECTS OF A 6-WEEK NEUROMUSCULAR TRAINING PROGRAM ON KNEE JOINT MOTOR CONTROL DURING SIDECUTTING IN HIGH-SCHOOL FEMALE ATHLETES

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1 ABSTRACT THE EFFECTS OF A 6-WEEK NEUROMUSCULAR TRAINING PROGRAM ON KNEE JOINT MOTOR CONTROL DURING SIDECUTTING IN HIGH-SCHOOL FEMALE ATHLETES by Justin Phillip Waxman The purpose of this study was to implement a 6-week prophylactic neuromuscular training program prior to the athletes upcoming competitive season and to experimentally analyze the neuromuscular adaptations elicited by this training during a standardized sidecutting maneuver. Sixteen competitive female athletes aged 15 ± 1 year participated in the study. A pre- and posttest was performed within 1-week of the start and end of the intervention, respectively. Neuromuscular activity at the knee joint and initial ground contact time (IC) were recorded using surface electromyography (EMG) and footswitches, respectively. Neuromuscular activity was obtained at 10- and 50-ms time-intervals both before and after IC. Results revealed a significant increase in the neuromuscular activity of the biceps femoris muscle (P < 0.01) 10-ms after IC, while quadriceps activity remained unchanged. In conclusion, the selective increase in the lateral hamstring muscles may reduce the risk of anterior tibial shear force and internal tibial rotation, potentially decreasing non-contact ACL injury risk among female athletes.

2 THE EFFECTS OF A 6-WEEK NEUROMUSCULAR TRAINING PROGRAM ON KNEE JOINT MOTOR CONTROL DURING SIDECUTTING IN HIGH-SCHOOL FEMALE ATHLETES A Thesis Submitted to the Faculty of Miami University in partial fulfillment of the requirements for the degree of Master of Science Department of Kinesiology and Health by Justin Phillip Waxman Miami University Oxford, OH 2012 Advisor Mark S. Walsh, PhD Reader Rose M. Ward, PhD Reader William Berg, PhD

3 Table of Contents Page Number List of Tables. iv Last of Figures... v Introduction... 1 Proposed Mechanisms of Injury for Female Athletes... 2 External Loads Applied to the Knee Joint. 3 Knee Joint Musculature. 4 Anticipatory Effects on Knee Joint Loading. 6 Early Work on Neuromuscular Activation in Anticipated & Unanticipated Conditions.. 7 Biomechanical & Neuromuscular Gender Differences. 10 Plyometric Training Programs.. 14 Adaptation Mechanisms Elicited by Plyometric Training Research Question. 25 Materials & Methods. 27 Participants 27 Procedures. 27 Sidecutting Maneuver Prophylactic Neuromuscular Training Program Instrumentation.. 29 EMG Signal Treatment Normalization To Peak Amplitude Obtained During Sidecutting 31 Statistical Analysis Results Discussion.. 32 Limitations. 39 Conclusion. 41 ii

4 References. Appendix A: Consent Form... Appendix B: Dynamic Warm-up... Appendix C: Electrode Placement iii

5 List of Tables Page Number Table 1: Jump Training Program 50 Table 2: Glossary of Jump Training Exercises Table 3: Dominant Leg: Neuromuscular Activation During Sidecutting Normalized to Peak Amplitude During Sidecutting (% of max) 52 Table 4: Non-Dominant Leg: Neuromuscular Activation During Sidecutting Normalized to Peak Amplitude During Sidecutting (% of max).. 53 Table 5: Co-Contraction Ratios 54 Table 6: Medial-to-Lateral Hamstring Ratios iv

6 List of Figures Figure 1: Diagram of experimental setup (not to scale) 56 Figure 2: Neuromuscular activity recorded for the medial (ST) and lateral (BFcl) hamstring muscles before and after prophylactic neuromuscular training (Mean average EMG amplitude in the 10-ms time interval after IC) 57 Page Number v

7 The Effects of a 6-Week Neuromuscular Training Program on Knee Joint Motor Control During Sidecutting in High School Female Athletes INTRODUCTION Knee injuries are a common concern amongst athletes at all levels of competition and are currently the largest single problem in the field of orthopedic sports medicine (Renstrom et al., 2008). Such injuries can lead to extreme discomfort, the discontinuation of an athlete s athletic season, increased susceptibility to osteoarthritis, and even surgery. The most common type of injury to the knee is a rupture of the anterior cruciate ligament (ACL), which accounts for more than half of the reported injuries and can occur in both contact and noncontact situations. In the United States alone, approximately 250,000 ACL injuries occur on a yearly basis (Silvers & Mandelbaum, 2011), which correlates to a 1 in 3,000 chance that an individual in the general population will sustain an injury to the ACL. However, caution should be taken when interpreting such data because these numbers are presently limited by the absence of any standardized reporting system for the general population. Although registries exist for injuries sustained by United States college and high school athletes, these registries account for only a small percentage of the total number of injuries in a given year (NCAA, 2002; NFHS, 2002). Furthermore, it has been found that more than 70% of injuries to the ACL are sports related (Johnson, R. J., 1988; Smith, Livesay, & Woo, 1993) and in addition to the aforementioned negative consequences of sustaining an injury to the ACL, the financial burden is also rather significant. When taking the cost of imaging, reconstructive surgery, postoperative bracing, and rehabilitation into account, the average annual cost has been found to exceed 2 billion dollars, making this a matter of great concern for public health. Following the institution of Title IX of the Educational Assistance Act in 1972, the number of young females participating in sports increased 900% in high school athletics (NFHS, 2002) and 500% in collegiate athletics (NCAA, 2002). Prior to the enactment of this legislation, a gender disparity in knee injuries had not been documented. However, since Title IXs inception and the dramatic increase in female sports participation, female athletes have been found to be at an increased risk of sustaining a non-contact ACL injury compared to their male counterparts. Over the past decade, it has been well documented that female athletes experience a 3- to 8-fold higher incidence of non-contact ACL rupture when compared with male athletes competing in the same 1

8 sports (Agel, Arendt, and Bershadsky, 2005; Arendt & Dick, 1995; Hewett et al, 1999;) and this phenomenon is most often seen in soccer, basketball, team handball and volleyball. Thus, the focus of research over the past decade has been to investigate the underlying mechanisms that may contribute to the gender disparity and to develop prevention strategies aimed at reducing the incidence of this injury. Proposed Mechanisms of Injury for Female Athletes A variety of mechanisms have been proposed as contributing to the increased non-contact ACL injury rate in female athletes and these factors can be categorized as anatomical, hormonal, or biomechanical, in addition to other extrinsic factors. Anatomical factors include the small crosssectional area of the ACL, a narrower femoral (intercondylar) notch, a greater quadriceps angle (Q-angle), and increased knee joint laxity in female athletes. To the best of my knowledge however, these proposed anatomical risk factors have not been found to have a direct correlation with an increased risk of non-contact ACL injury (Griffin et al., 2000). Hormones including estrogen, progesterone, and relaxin, which fluctuate throughout the female menstrual cycle, may also contribute to the increased injury rate by affecting the neuromuscular and musculoskeletal systems (Hewett, 2000). This may result in increased knee joint laxity or altered muscle strength in female athletes. Controversy exists regarding the findings from investigations of hormones on injury risk and these factors, along with anatomical gender-differences, can be difficult to affect. On the other hand, biomechanical or neuromuscular mechanisms may warrant the most attention because neuromuscular mechanisms may prove to play the largest role in the gender differences contributing to ACL injuries. Therefore, the focus of the current review of the literature will focus on biomechanical/neuromuscular aspects only. While the ACL can be injured from a contact blow to the knee, it has been reported that 60-80% of ACL injuries occur in non-contact situations (Brukner & Khan, 2006). Biomechanical investigations on non-contact injury mechanisms have found that these injuries all appear to share common features in that they typically involve sudden changes in direction combined with acceleration or deceleration of the body, such as when landing from a jump or cutting in order to quickly evade an opponent (Boden, Dean, Feagin, & Garrett, 2000; Noyes, Mooar, Matthews, & 2

9 Butler, 1983; Ryder, Johnson, Beynnon, & Ettlinger, 1997). Additionally, these non-contact injuries to the ACL all tend to occur near foot-strike (initial ground contact), when the quadriceps muscles are eccentrically activated to resist knee flexion. In order to further understand the relationship between these high-risk maneuvers and the increased injury rate in female athletes, it is important to first measure the external loads applied to the knee during movements that challenge the integrity of the joint because, as Whiting and Zernicke (1998) stated, only when the causal relations between applied forces and resultant injury are established and understood can appropriate programs of intervention and prevention be designed and implemented (as cited in Besier, Lloyd, Cochrane, & Ackland, 2001a). External Loads Applied to the Knee Joint Early cadaveric studies have measured the loads experienced by the ACL and PCL under combined loads applied to the tibia throughout a range of knee angles. The combination of loads included knee flexion, varus/valgus (adduction/abduction, respectively), and internal/external rotation. A combination of externally applied internal rotation and anterior tibial force resulted in the highest load on the ACL (Markolf, Wascher, & Finerman,1993). Markolf et al. (1993) also found that purely internal rotational moments, as well as varus/valgus moments in combination with anterior tibial force, resulted in large loads placed on the ACL. In addition, it was found that the ACL experienced larger loads at more extended knee angles, particularly between 10 and 40 of knee flexion. These findings, regarding external loads on the knee in cadaveric studies, laid the groundwork in helping researchers understand a primary mechanism behind non-contact ACL injury and led to later studies, which aimed to investigate such loads during dynamic tasks. In 2001(a), Besier, Lloyd, Cochrane, and Ackland conducted a study which investigated the external loads applied to the knee joint during dynamic cutting tasks and assessed how these loads might contribute to ligament loading. A total of four tasks were analyzed: a straight run (RUN), a side-cut at 30 (S30) and at 60 (S60), and a crossover cut at 30 (XOV), stepping off the right foot. It was found that although the flexion loads were large, there was almost no difference in applied flexion moments during the different tasks. However, significant differences were found in varus/valgus (VV) and internal/external (IE) rotational loading. 3

10 Previously, a cadaveric study found that ligament damage occurred within 35- rotation and within 125- (Piziali, Nagel, Koogle, & Whalen, 1982). The peak IE and VV moments measured during the cutting tasks of Besier et al. (2001a) were within the range measured by Piziali et al. (1982) and were coupled with large external flexion loads, suggesting that these tasks are capable of putting the ACL at increased risk for injury. In addition, it was found that the greatest potential for tension development in the ACL occurs during the side-cut task at initial ground contact and at ± 10% of the peak resultant ground reaction force (GRF). During the phases of initial ground contact and ± 10% of peak GRF, the knee experiences combined loads of anterior tibial shear force, internal rotation, and valgus moments, while the knee is between 30 and 40 of knee flexion (Besier et al., 2001a). These VV and IE moments are believed to be primarily responsible for placing the knee joint ligaments at a higher risk for injury. Compared with straight running, the external loads of flexion, valgus, and internal rotation during sidecutting tasks have the ability to dramatically increase the loads experienced by the ACL and medial collateral ligament (MCL). Because the loads produced by sidecutting are of a sufficient magnitude to rupture these ligaments, it then seems essential to direct attention to the musculature surrounding the knee joint. If the muscles surrounding the joint are unable to adequately support these combined loads, it is likely that an athlete will suffer from a ligament injury as a result. Knee Joint Musculature The main musculature surrounding the knee joint consists of the quadriceps and hamstring muscle groups, and the balance of muscular power and recruitment pattern between these two groups play a critical role in functional knee stability. The quadriceps muscle group is made up of four muscles that join together and form the quadriceps tendon. This tendon then connects the muscle group to the patella, which in turn connects to the tibia via the patellar tendon. The primary functional role of the patella is to allow extension of the knee; the patella increases the mechanical leverage that the tendon can exert on the tibia by increasing the angle at which it acts. Thus, contraction of the quadriceps muscle group pulls the patella upwards, leading to 4

11 extension of the knee. In contrast, the hamstring muscle group functions to flex the knee joint as well as provide stability on both the medial and lateral side. In 2000, Colby et al. looked to qualitatively characterize the activation of the quadriceps and hamstring muscles, as well as knee flexion angles, during the eccentric motion of high-risk maneuvers including sidecutting, crossover cutting, stopping, and landing. A qualitative analysis demonstrated increasing activity of the quadriceps muscles at foot-strike in all maneuvers studied. All maneuvers except landing were characterized by increasing hamstring activity before foot-strike and decreasing or steady hamstring muscle activation at and after foot-strike. The quadriceps muscles generally peaked during mid-eccentric motion while the minimum hamstring muscle activation occurred just after foot-strike and the maximum difference between quadriceps and hamstring muscle activation occurred after the minimum hamstring activation (just after foot strike), but before the peak quadriceps muscle activation (mid-eccentric motion). It was also found that foot-strike occurred at an average of 22 of knee flexion for all maneuvers studied and that the level quadriceps muscle activation frequently exceeded values recorded during maximal isometric contraction. Colby et al. (2000) then argued that the minimal hamstring activation, coupled with the forces generated by the quadriceps muscles at the knee, could result in significant anterior displacement of the tibia at the knee and thus, lead to ACL injury. Studies prior to Colby et al. (2000) have sought to examine the role of the quadriceps pulling the tibia anteriorly, causing increased stress on the ACL at low knee flexion angles. Arms et al. (1984) and Renstrom et al. (1986) both reported that quadriceps muscle activity is capable of significantly straining the ACL at knee flexion angles ranging from 0 to 45 (as cited in Colby et al., 2000). Smidt et al. (1973) measured the maximum isometric contraction of the quadriceps muscles during knee flexion and extension and found that they exert significant anterior shear force on the tibia when the knee is flexed between 5 and 60 (as cited in Colby et al., 2000). Additionally, research on cadaver specimens has found that anterior tibial displacement resulting from quadriceps muscle force is greatest at 30 to 45 in knees with an intact ACL and 20 to 25 in knees with a sectioned ACL (Shoemaker et al., 1993, as cited in Colby et al., 2000). This information, coupled with the finding of Colby et al. (2000) that foot-strike occurred at an 5

12 average angle of 22 knee flexion during high-risk maneuvers, clearly demonstrates that the knee is positioned at an angle that will allow the quadriceps muscles to strain the ACL. This finding has been further supported by other research, which has reported average knee flexion angles at landing of 32.8 during similar tasks (Besier et al., 2001a). Supplementary findings from Besier et al. (2001a) suggested that the athletes cutting technique may be more important than running speed or cutting angle in regard to the potential for ligament injury. Half of the subjects in this study performed the sidecutting task with a net varus moment applied to the knee (varus group), whereas the other half experienced a net valgus moment (valgus group). The varus group was able to maintain a greater speed and achieve a greater cutting angle during the side-cut compared to the valgus group. In a separate study that examined female knee joint kinematics during a sidecutting maneuver, McLean, Neal, Myers, & Walters (1999) concluded that the athlete s level of experience appears to be a major contributing factor to the degree of consistency in the performance of the maneuver. This evidence, along with the differences observed in VV moments during sidecutting, may suggest that cutting technique alone can affect the external load placed on the knee joint. It is also important to keep in mind that the tasks performed in these studies were carried out in a laboratory setting and that the athletes in these investigations were made aware of the maneuver that was to be executed prior to carrying out the movement. This is important because it is possible that the subjects planned their postural and movement strategies in advance, which may cause the resultant moments to be different from what might be experienced in a game situation. During game situations, athletes are typically forced to react to external stimuli with very little time for decision-making; therefore, it is likely that preplanned cutting maneuvers in the laboratory are not a true reflection of the loads applied to the knee joint during sporting situations (Besier, Lloyd, Ackland, & Cochrane, 2001b). Anticipatory Effects on Knee Joint Loading In order to further investigate the anticipatory effects on knee joint loading, Besier et al. (2001b) conducted a study that compared the external loads applied to the knee joint during preplanned (PP) and unanticipated (UN) running and cutting maneuvers and related their findings to the 6

13 potential for non-contact knee ligament injury. Findings from their previous investigation (2001a) were used as baseline data and compared with the results from conditions that would more likely represent loads experienced during game situations. Results demonstrated that the external VV and IE moments applied at the knee joint during UN cutting maneuvers were as great as two times the magnitude of those experienced during PP conditions. Besier et al. (2001b) then suggested that such large increases in VV and IE moments might significantly increase the loading of the knee joint ligaments. It was then hypothesized that inappropriate postural adjustments were responsible for the increased loading during where little time is given to prepare for the task. Based on this hypothesis, it was advocated that future research focus on the nature of such postural adjustments, as well as muscle activation patterns, during PP and UN cutting maneuvers in order to determine whether or not the musculature surrounding the knee joint is activated in such a way that would counter the increased external loads and protect the knee joint ligaments. Early Work on Neuromuscular Activation in Anticipated and Unanticipated Conditions Research has established that the muscles surrounding the knee have the ability to support the large external loads applied to the joint and reduce the potential for ligament loading due to their anatomical moment arms (Lloyd, 2001; Lloyd & Buchanan, 2001). When destabilizing forces are anticipated, the Central Nervous System (CNS) is capable of adjusting muscle activation patterns to oppose these forces, which supports the notion that anticipatory postural adjustments are planned in detail (Besier et al., 2001b). This notion has been supported further by a study that has examined neuromuscular activation during a dynamic landing task. Prior to landing, the hamstring muscles become activated, which is believed to function to counter the anterior translation of the tibia with respect to the femur that occurs just after landing in order to protect the ACL (Cowling & Steele, 2001). Conclusions from previous investigations lead Besier, Lloyd, and Ackland to conduct an additional study in 2003, which aimed to investigate the activation patterns of muscles surrounding the knee joint during PP and UN running and cutting tasks with respect to the external moments applied to the joint. Using the same experimental design that had been used 7

14 previously (2001b), Besier et al. (2003) included the use of electromyography (EMG) in order to assess the neuromuscular activation of the following 10 muscles: semimembranosus (ST), biceps femoris (BFcl), satrorius (SR), tensor fascia latae (TFL), gracilis (GRA), vastus lateralis (VL), vastus medialis (VM), rectus femoris (RF), medial gastrocnemius (MG), and lateral gastrocnemius (LG). Prior to data collection, three main questions were asked. First, does the CNS modify activation patterns to stabilize the knee for increases in VV and IE loading during sidecutting tasks, compared with a straight run, even when external flexion moments are the same? Second, does the CNS use a directed activation of medial/lateral and internal/external rotation muscle groups according to the expected moments at the knee in PP cutting tasks, or is there a generalized co-contraction strategy? Third, does the CNS adopt a more generalized cocontraction pattern of muscle activation, compared with a directed muscle activation strategy, during cutting tasks that are unanticipated compared to when these tasks are preplanned? With the three fundamental questions listed above and knowledge from their prior research (Besier et al., 2001a) about external loads experienced at the knee joint, Bessier et al. (2003) developed five research hypotheses: (1) Average muscle activation will increase in the cutting tasks compared with a straight run (RUN), in line with increased VV and IE loads. (2) During PP sidestepping, activation of medial and external rotation muscles will increase compared to the RUN task, to counter the applied valgus and internal rotation moments at the knee. (3) During a crossover cut (XOV), activation of lateral and internal rotation muscles will increase compared to the run task, to counter the applied varus and external rotation moments at the knee. (4) During UN conditions, the CNS will adopt a generalized co-contraction strategy, as opposed to directed activation patterns to counter the VV and IE loads. (5) Average muscle activation during the UN cutting tasks will be greater than that found in the PP cutting condition to counter the large increase in VV and IE loads. To answer the first hypothesis, it was found that the CNS does appear to activate the knee muscles to stabilize the knee joint in VV and IE rotation, in addition to FE during dynamic tasks. In their previous study, Besier et al. (2001a) showed that VV and IE moments increased during cutting tasks, compared to the RUN, whereas FE moment did not change. Based on this, the 8

15 increased activation may be preplanned by the CNS in order to counter the increased loading in the VV and IE directions. During the pre-contact phase (PC) of the sidecut, the level of activation was scaled in anticipation to the size of the expected VV and IE moments, but not the FE moments, and the magnitude of the average muscle activation during the PP tasks were larger when greater VV and IE moments were expected. These findings, which suggest that the CNS scales the activation level for stabilization of the knee joint in directions other than flexion and extension, were rather significant in expanding on the depth of previous work in this area. In response to the second, third, and fourth hypotheses, the results of this study found that combinations of co-contraction and selected muscle activation are present during a dynamic sidecutting task. During PP conditions, there was selective activation of medial muscle groups in order to counter the external valgus load at the joint and selected activation of the lateral hamstring to counter the applied internal rotation moments, which supports the second hypothesis. In addition to a selected activation during PP sidecutting, results also demonstrated co-contraction of the flexors and extensors. This finding supports previous research done by Lloyd and Buchanan (1996), suggesting that by using both of these strategies, the CNS is capable of altering a set of activation patterns at the knee to suit the external load applied to the joint during sidecutting (as cited by Besier et al. 2003). Results failed to support the third hypothesis; although large varus moments are experienced during the XOV, a generalized cocontraction strategy was used, causing no increase in muscle activation. A previous study conducted by Toussaint et al. (1998) has demonstrated that the level of experience in performing a particular skill can alter the preprogrammed movements necessary to perform the skill (as cited by Besier et al. 2003). Besier et al. (2003) suggest that their findings regarding the XOV task may be due to unfamiliarity, resulting in a preprogrammed co-contraction strategy that may not be as finely tuned as the subject s strategy used during sidecutting. A generalized co-contraction strategy was also found during unanticipated sidecutting maneuvers, which supports the fourth hypothesis. The final hypothesis was only partially supported by the results. There was an observed increase in muscle activation during the unanticipated sidecutting task when compared to the preplanned 9

16 condition; however, the relative increase in muscle activation was small (10-20%) when compared with an increase in valgus moments of ~70% and an increase in internal rotational moments of ~90% during the unanticipated sidecutting task. Besier et al. (2003) suggest that this mismatch between changes in the preprogrammed muscle activation strategy and the increased VV and IE moments may be responsible for placing large enough loads on the knee joint ligaments to cause injury. Furthermore, although other factors also play a role, it appears that the risk of noncontact ACL injury dramatically increases in unanticipated conditions. As mentioned previously, female athletes are at a much higher risk of non-contact ACL rupture when compared to their male counterparts competing in the same sports. One notable limitation of the majority of the research discussed thus far is that the participants used in these studies have been predominantly male. In order to gain a more concrete understanding of the underlying biomechanical/neuromuscular mechanisms that put females at increased risk, other studies have drawn attention to how gender impacts these results. Biomechanical and Neuromuscular Gender Differences In order to assess kinetic and kinematic differences between genders, Malinzak, Colby, Kirkendall, Yu, and Garrett (2001) compared three-dimensional knee joint motions end EMG activity of the quadriceps and hamstring muscle groups between male and female recreational athletes during running, crossover cutting, and sidecutting. It was found that female and male athletes, on average, had different knee motion patterns in the selected maneuvers. Specifically, female athletes were found to have decreased knee flexion angles, increased knee valgus angles, increased quadriceps muscle activation, and decreased hamstring muscle activation when compared to males. These results indicate that female athletes have knee motion patterns that frequently bring them closer to body alignment positions that are likely to result in non-contact ACL injury. In all three tasks, female subjects displayed smaller knee flexion angles compared to males, which suggest that the knee is in a position that would allow the quadriceps to exert a greater amount of anterior tibial shear force than at greater knee flexion angles; thus, leading to increased stress on the ACL. The finding that females also exhibited increased knee valgus suggests a further increase in risk for injury. Additionally, the finding that female subjects had 10

17 higher quadriceps activation and lower hamstring activation in the selected tasks than males may suggest inadequate neuromuscular activation to protect the ligaments surrounding the joint. It should be noted however, that all of the tasks performed in this study were anticipated. In order to further develop our understanding of gender differences in situations that more closely mimic competitive play, Landry et al. (2007) investigated these differences under unanticipated conditions. Landry et al. (2007) compared measures of muscular strength, hip, knee, and ankle kinematic and kinetic waveforms, as well as muscle activation waveforms for the quadriceps, hamstrings, and gastrocnemii muscles during the stance phase of an unanticipated side-cut maneuvers between male and female adolescent soccer athletes. These parameters were chosen because it was hypothesized that gender and mediolateral muscle site differences existed in both the magnitude and temporal characteristics of such waveforms, and that determining these differences would help better explain the risk factors related to the higher prevalence of noncontact ACL injury in female athletes. In support of the findings of Malinzak et al. (2001), Landry et al. (2007) also identified neuromuscular and biomechanical differences between genders. During the unanticipated side-cut, female athletes demonstrated greater lateral and medial gastrocnemius, and rectus femoris muscle activation magnitudes during the stance phase of the side-cut. The stance phase was defined as the instant the foot made contact with the force plate to the instant the foot was removed from the force plate. Female athletes also exhibited smaller hip flexion angles and moment magnitudes throughout the stance phase compared with male athletes. Furthermore, hip adduction and internal rotation, knee valgus, and ankle eversion moment differences were also noted during the initial 10% to 20% of the stance phase; the time frame in which ACL injuries are most likely to occur (Besier et al., 2001a). When evaluating muscular strength and flexion/extension strength ratios between genders, no differences were found after normalizing the generated moments to body weight and height (Landry et al., 2007). The differences between genders in neuromuscular activation magnitude for the medial and lateral gastrocnemii during sidecutting were first identified by Landry et al. (2007). Female 11

18 subjects were found to have greater lateral and medial activity compared to the male subjects during the early- and mid-stance phases of the sidecutting maneuver. When analyzing female subjects only, an imbalance was found for the entire duration of the movement, with the lateral gastrocnemius being more active than the medial. Landry et al. (2007) suggested that this higher activity in the gastrocnemii muscles might be necessary to help the quadriceps and hamstrings stabilize and stiffen the female knee joint. It was noted however, that although this could help protect the knee, it might actually increase ACL strain because other studies have demonstrated that the contraction of these muscles alone, or in combination with the quadriceps, are capable of increasing the load on the ACL (O Connor, J. J., 1993; as cited in Landry et al., 2007, p.1898). The finding that female athletes also demonstrated greater rectus femoris muscle activity than the male athletes may provide support for the latter, in that Arms et al. (1984) and Renstrom et al. (1986) both reported that quadriceps muscle activity is capable of significantly straining the ACL at knee flexion angles ranging from 0 to 45 (as cited in Colby et al., 2000). During the assessment of the unanticipated side-cut sagittal plane kinematics, differences between genders were found at the hip, but not at the knee (Landry et al., 2007). McLean, Lipfert, and Van Den Bogert (2004) also found females to have smaller hip flexion, in addition to smaller knee flexion, angles during the side-cut than males. While these two studies are in agreement with respect to hip flexion angles, the findings of Landry et al. (2007) were unable to support such differences at the knee joint. It is worth mentioning, however, that Malinzak et al. (2001) also found decreased knee flexion angles in female athletes. Cutting with less flexion at the hip or knee would place the female athlete in a more erect posture; these small flexion angles have been shown to generate higher impact forces, which would place greater stress on the joint in addition to increased susceptibility for experiencing anterior tibial shear from quadriceps contraction. The research that has been discussed thus far, related to gender differences during high risk sporting maneuvers, has demonstrated that females may have gender-related neuromuscular imbalances in muscle contraction patters that can lead to increased risk for ACL injury. Results from video analyses of ACL injuries in female team handball and basketball athletes have 12

19 identified two main injury mechanisms. The most common situation (12 of 20 injuries) occurred during a plant-and-cut movement (i.e., sidecutting), which was accompanied by forceful valgus collapse and internal or external rotation while the knee was close to full extension (Olsen, Myklebust, Engebresten, & Bahr, 2004). The second situation (4 of 20 injuries) was during a one-legged jump-shot landing, which also occurred with a forceful valgus collapse and external rotation while the knee was near full extension (Olsen et al., 2004). In additional work, Krosshaug et al. (2007) estimated that the time of injury occurred milliseconds after initial ground contact. All of the research regarding the loads experienced at the knee joint seem to be in support of these findings and provide a multitude of evidence for the theory that an inadequate mismatch between the increased loads and activation of the surrounding knee joint musculature can lead to ACL rupture. In order to further evaluate the gender differences in muscle activation strategies when performing a high risk maneuver, Myer, Ford, and Hewett (2005) looked to examine neuromuscular control in female athletes in the absence of a high velocity or high load. It was thought that an unbalanced or low ratio of medial to lateral quadriceps recruitment, combined with increased recruitment of the lateral hamstring, would compress the lateral aspect of the joint while leaving the medial aspect open and increase anterior shear force on the tibia. This would result in increased potential for dynamic valgus and increased stress on the ACL. Myer et al. (2005) hypothesized that the ratio of medial to lateral quadriceps activation would be decreased in female athletes and that female athletes would demonstrate increased lateral quadriceps activation compared to males. Additionally, it was hypothesized that the subjects anatomical Q- angle would not correlate to EMG measures in females or males. Results of the present study found that female athletes did indeed demonstrate a decreased ratio of medial to lateral quadriceps recruitment compared to males. This decreased ratio of medial quadriceps recruitment was thought to be related to decreased control of varus/valgus forces at the knee. Decreased medial knee joint compression may limit the passive resistance to dynamic knee valgus, which would make the female knee more susceptible to medial femoral condylar lift-off from the tibial plateau and increase the loading on the ACL when decelerating from a 13

20 landing or cutting maneuver (Myer et al., 2005). The increased lateral quadriceps recruitment also results in increased anterior shear force, which directly loads the ACL. The noted neuromuscular activation strategies of female athletes in the present study is thought to limit the effectiveness of the active neuromuscular control systems to work synergistically with the passive joint restraints (knee-joint ligaments) and create dynamic knee joint stability (Myer et al., 2005). Therefore, the results of Myer et al. (2005) suggest that female athletes utilize different quadriceps recruitment patterns compared to males when performing a maneuver that mimics a main mechanism of ACL injury. Additional findings of Myer et al. (2005) demonstrated that there was not a significant correlation between men and women s anatomical Q-angle and non-contact ACL injury. Similar findings were also noted in a study performed by Endsley, Ford, Myer, Slauterbeck, and Hewett (2003) which found that Q-angle did not correlate to measures related to dynamic knee stability (as cited in Myer et al., 2005). Although the exact cause of ACL injury in female athletes may be due to a combination of factors, this information provides evidence that there should be an increased focus on the neuromuscular components of ACL injury, because these aspects are modifiable. To that end, the implementation of prophylactic neuromuscular training programs for female athletes has been shown to increase active knee stabilization in the laboratory and decrease the incidence of non-contact knee injury during competition. This evidence suggests that prophylactic training may facilitate neuromuscular adaptations, which can teach athletes to utilize joint stabilization patterns that employ safer neuromuscular recruitment strategies and reduce the risk of injury in at-risk populations. Plyometric Training Programs Jump-training programs, which incorporate stretching, plyometric exercises, and weight training, have been advocated to increase athletic performance and decrease injury risk in sports that involve jumping and cutting. Studies published as early as 1988 have reported alterations in athletic performance through the implementation of such programs developed by high school, collegiate, and Olympic sports teams; however, it was not known whether such programs affected jumping and landing biomechanics (Hewett, Stroupe, Nance, & Noyes, 1996). The first study to examine alterations in jumping and landing mechanics of the lower extremity, following 14

21 the implementation of a plyometric training program, was published in The purpose of this study was to test the effects of a jump-training program on the mechanics of landing and on strength of the lower extremity musculature in female athletes involved in jumping sports. In this study, Hewett et al. (1996) employed a six-week training program that was designed to decrease landing forces by teaching neuromuscular control of the lower limb during landing and to increase joint stability by increasing the strength of the knee joint musculature. These parameters were then compared before and after training with those of male athletes. Following the six-week training program, ten of eleven female participants displayed decreased peak landing forces by an average of 22% (P = 0.006)(Hewett et al., 1996). The noted decrease in landing forces is important because this directly translates to a decrease in the forces experienced at the joints of the lower extremity (Besier et al., 2001a). In addition, it was found that peak landing forces were significantly influenced by adduction and abduction moments at the knee. In the present study, seven of the female subjects had higher peak adduction moments, than abduction moments, at landing (adduction-dominant), while the other four female subjects had higher abduction moments at landing (abduction-dominant) (Hewett et al., 1996). In adduction-dominant subjects, the adduction force decreased with training from 3.4 ± 1.6 to 2.1 ± 1.0 %BW height. Abduction-dominant subjects also showed a similar decrease from 4.0 ± 1.8 to 1.9 ± 1.1 %BW height (P < 0.01). A multiple regression analysis incorporating flexion angles, flexion and extension moments, and adduction and abduction moments at the knee, hip, and ankle demonstrated that adduction and abduction moments at the knee were the only significant predictors of peak landing forces (P = 0.006). The observed decrease suggests that training may have altered the athletes neuromuscular control of the lower extremity in the coronal plane, which may be caused by changes in adductor and abductor muscle contraction patterns (Hewett et al., 1996). A decreased adduction or abduction moment tends to decrease the risk of femoral-condylar liftoff from the tibial plateau and associated knee ligament injury. An increase in hamstring muscle peak torque and power was also observed following the six-week training period, which may increase the ability for this musculature to protect the knee joint. 15

22 When assessing strength, Hewett et al. (1996) found significant differences in the hamstring muscles before and after training. Isokinetic peak torque increased 26% in the non-dominant leg (P = 0.012) and 13% in the dominant leg (P = 0.094). Isokinetic average power of the hamstring muscles also increased; the dominant leg increased 44% and the non-dominant increased 21%. Additionally, the hamstring-to-quadriceps muscle peak torque ratio increased 13% for the dominant side and 26% on the non-dominant side. After training, the female athletes had reached a level equivalent to that of untrained male subjects. To summarize, ten of the eleven subjects were shown to effectively decrease their peak landing forces, which directly translates to decreased forces experienced at the knee joint. The multiple regression analysis showed that peak landing forces were significantly influenced by adduction and abduction moments at the knee joint. The plyometric training program effectively decreased adduction and abduction moments at the knee during landing, which suggests that this type of training may alter muscular control of the lower extremity in the frontal (coronal) plane. Hewett et al. (1996) suggested that this noted decrease is most likely reflected by changes in contraction patterns of the adductors and abductors at the knee. Although these moments had a lower magnitude than the knee flexion moments at landing, they showed a significant relationship with peak forces, and flexion moments did not. Previous biomechanical studies on the relationship between varus/valgus stress and injury risk have demonstrated that a valgus (abduction) moment greater than 35 N-m causes pain in female athletes (Pope et al., 1979 as cited by Hewett et al., 1996) and stresses greater than 29 N-m place major stress on the collateral ligaments of the knee (Markolf et al., 1978 as cited by Hewett et al., 1996). In Hewett et al. (1996), the abduction (valgus) and adduction (varus) moments were 42 and 36 N-m before training and 20 and 22 N-m after training, suggesting a reduced risk of injury to the ligaments of the knee. The increase in knee flexor peak torque and power following training was rather significant over the 6-week period. The program also brought the female athletes from a hamstring-to-quadriceps ratio that was significantly lower than the male subjects (51% versus 65%, respectively) up to an equivalent value. 16

23 As previously discussed, the plyometric, stretching, and strength training program developed by Hewett et al. (1996) was shown to decrease peak landing forces by decreasing abduction/adduction moments at the knee. The program also significantly increased hamstring muscle power and strength, increased hamstring-to-quadriceps peak torque ratios, and decreased side-to-side hamstring muscle strength imbalances. Due to the biomechanical evidence of this 1996 study, Hewett et al. (1999) hypothesized that this program might decrease injury rates in female athletes. Additionally, if this proved to be valid, similar neuromuscular training programs could be used to alter the current methods in preseason training and significantly reduce the number of female athletes injured each year. Hewett et al. (1999) decided to focus on female soccer, volleyball, and basketball athletes for the study, due to the high amount of jumping and cutting involved in these sports. All together, 43 sports teams from 12 Cincinnati area high schools were included in the study. Fifteen girl s teams (366 female athletes) participated in the six-week preseason neuromuscular training program (trained group), another fifteen girls teams (463 female athletes) did not receive the neuromuscular training and went about their regular preseason routines (Untrained group), while thirteen boys teams (434 untrained boys) served as a control group. All athletes were given preseason screening questionnaires regarding prior injuries and how long they had been participating in their particular sport. Certified athletic trainers then submitted weekly reports on the number of injuries that occurred during any given week, along with the game and practice injury risk exposures; A injury risk exposure has been defined by Hewett et al. (1999) as one athlete participating in one practice or game. These reports were completed each week throughout the entire sports season for each team. After some subject s data were excluded for various reasons, investigators recorded 14 serious knee injuries out of 1263 subjects (Hewett et al., 1999). All of the injuries reported in this study occurred during non-contact situations. This was the first prospective study to report the effects of neuromuscular training on knee injury in female athletes who are involved in high-risk sports. The incidence of serious knee injury was found to be 2.4 to 3.6 times higher in the untrained group than the trained group, depending on whether or not the volleyball subjects were included 17

24 (Hewett et al., 1999). Untrained female athletes were found to be 4.8 to 5.8 times more likely than male athletes to suffer a knee injury, and trained female athletes were 1.3 to 2.4 times more likely than males to suffer a knee injury. These results indicate that neuromuscular training is likely to decrease the risk of injury in female athletes. Based on the finding of their previous study (1996), Hewett et al. (1999) asserted that neuromuscular training is an intervention that can have a biomechanical effect, such as decreased landing forces in adduction and abduction moments, as well as a physiological effect, such as decreased estrogen levels and increased hamstring-to-quadriceps ratios. With that said, Hewett et al. (1999) conclude that the decreased incidence of injury in trained athletes might be due to increased dynamic stability at the knee joint after training. The findings of Hewett et al. (1999) did not go unnoticed by other researchers. A few years after the study, Myklebust et al. (2002) conducted a similar study, but implemented a different training program. Myklebust et al. (2002) hypothesized that improving awareness of the knee position, balance, and cutting and landing technique could reduce the frequency of ACL injuries, due to the fact that most of these injuries in female athletes are noncontact. By implementing a neuromuscular training program that was designed to improve awareness and knee control during standing, cutting, jumping, and landing, Myklebust et al. (2002) aimed to assess the effectiveness of such a program on the incidence of ACL injury in elite female handball players. This intervention study covered three consecutive seasons of the three top divisions in the Norwegian Handball Federation. During the first season (control season), baseline data was collected on the incidence of ACL injuries. Then, an ACL injury prevention program was introduced before the start of the following two seasons (first & second intervention seasons). Myklebust et al. (2002) continued to register any incidences of injury throughout the two intervention seasons in order to assess the effectiveness of the prevention program. The main finding of this study was that there was a reduction in the incidence of ACL injuries (although not significant) from the control season to the second intervention season among the elite players who completed the training program. Myklebust et al. (2002) also found a significant reduction in the risk of noncontact ACL injuries. The fact that this intervention did 18

25 result in a decrease in the number of noncontact injuries suggests that preventative neuromuscular training programs can be quite effective. Myklebust et al. (2002) do note, however, that further research is needed in order to determine the effect of each component of such training programs on neuromuscular function and injury risk. More recent studies have focused on muscle activation and timing frequency during dynamic cutting tasks in order to further investigate the mechanism of noncontact ACL rupture. It has been established that plyometric and other neuromuscular training programs reduce the incidence of injury, but more research is needed to determine what adaptations occur as a result of such training and how these adaptations may reduce ones risk of future ACL injury. Adaptation Mechanisms Elicited by Plyometric Training Ebben et al. (2010) looked to assess gender differences in the magnitude and timing of hamstring and quadriceps activation, hamstring-to-quadriceps activation ratios, and hamstring-toquadriceps timing ratios both before and after foot contact during drop jumps and 45-degree angle side-cuts. The findings of this study demonstrated that there are gender differences in the magnitude and timing of hamstring and quadriceps muscle activation during functional movements that are very similar to those that cause ACL injuries. During the pre-contact phase of jump landings and sidecutting, men and women were found to be very similar with respect to the degree of activation of the hamstring and quadriceps muscles. Men, however, did produce greater lateral hamstring (LH) activation during the post-contact period of jump landings and sidecutting. This high degree of hamstring activation helps explain why men were also found to have higher hamstring-to-quadriceps activation ratios during the post-contact phase of the sidecutting maneuver. Men displayed earlier activation of the vastus lateralis (VL) and vastus medialis (VM) than women during the pre-contact phase of the jump and women demonstrated a longer duration of rectus femoris (RF) muscle burst during the post-contact phase of the cut. In regard to the hamstring-to-quadriceps activation ratio, men demonstrated a higher hamstring activation ratio than women for the post-contact phase of the cut. These results are consistent with the previous research. The averaged hamstring-to-quadriceps timing ratio demonstrated that women achieved earlier activation of the hamstring in the pre-contact phase of the side-cut compared to men while there was no other differences found for this variable. 19

26 Ebben et al. (2010) conclude that when compared to women, men tend to be hamstring dominant during the post-contact phase of the functional movements assessed in this study. They suggest that strength and conditioning interventions, such as resistance training and plyometrics, may offer the potential for increased rate and magnitude of hamstring muscle recruitment and should be evaluated using EMG during movements that are similar to those that cause ACL injuries in order to determine if gender differences in hamstring muscle function can be reduced. Much of the research that has been discussed up to this point has focused on the differences in landing mechanics, muscle activation, valgus alignment, and so on. Aware of the idea that hamstrings and quadriceps co-contraction may provide dynamic joint stabilization and potentially protect the knee during sports-related tasks, Myer et al. (2009) felt it necessary to investigate gender differences in muscle strength. In order to determine the association of quadriceps and hamstrings strength to ACL injury risk in female athletes, isokinetic knee flexion/extension strength was measured in 110 female and 22 male middle school, high school, and collegiate soccer and basketball players. Myer et al. (2009) hypothesized that decreased knee flexor and increased knee extensor strength would be observed in the female athletes who would subsequently suffer an ACL injury (FACL) compared to the uninjured female control (FC) and male control (MC) subjects. Results of this study demonstrated that the athletes who subsequently suffered ACL injury had decreased hamstrings strength, but not decreased quadriceps strength, compared to matched controlled males. On the other hand, female athletes who did not end up injured had decreased quadriceps strength, but not decreased hamstring strength, compared to matched male athletes (Myer et al., 2009). Myer et al. (2009) point out that although the median measurements in quadriceps and hamstrings strength were similar for FACL and FC groups, FACL subjects had 15% less hamstring strength compared to MC (95% CI, 1 to 27%; P = 0.04), and FC were not different than MC in hamstrings strength (P = 0.08) after adjusting for vertical jump height and accounting for the random effects arising from matching the groups. Conversely, the FC demonstrated a 10% decrease in quadriceps strength relative to MC (95% CI, 3 to 18%; P = 0.01). FACL subjects did not differ compared to the MC in quadriceps strength (P = 0.14). 20

27 The results of Myer et al. (2009) indicate that, when controlling for whole body power, FACL demonstrate similar relative quadriceps strength, but decreased hamstring strength, compared to uninjured male athletes. Myer et al. (2009) then suggested that decreased relative hamstring strength and recruitment might be a potential contributing mechanism to ACL injury in high-risk female athletes. Based on this assumption, it was suggested that: targeted neuromuscular interventions that increase relative hamstring muscle strength and recruitment may decrease injury risk and potentially increase performance in this population (p. 7). Although neuromuscular training programs, such as the ones developed by Myklebust et al. (2003) and Hewett et al. (1996), have been shown to reduce the incidence of non-contact ACL injuries in female athletes, the underlying neuromuscular adaptation mechanisms elicited by this type of training are largely unknown. Under the assumption that neuromuscular training programs may actually result in a remodeling of existing motor programs towards movement and activation patterns that potentially reduce ACL strain, Zebis et al. (2008) investigated the effect of such training on the specific neural activation patterns of the hamstring and quadriceps muscles during a standardized sidecutting maneuver. It was hypothesized that neuromuscular training would significantly change the pattern of neuromuscular activation for the lower limb muscles in a manner that potentially reduces specific risk factors predisposing for ACL injury among female elite team handball and soccer players. To test this hypothesis, neuromuscular activity and knee joint angles were measured by way of surface electromyography (SEMG) and electrogoniometry both before and after training. Additionally, a test-retest was implemented during a 6-month control season in order to separately evaluate the reproducibility of EMG data. Twenty female elite handball and soccer players (age, 26 ± 3 years; height, 174 ± 6 cm; weight, 70 ± 9 kg) were put through a 12-month injury prevention program. The program included 6 levels, each consisting of 3 exercises. Each of the 6 levels had to be followed 2 times a week (20 min/session) for 3 weeks before progressing to the next level; once each level had been completed (18 weeks), the 6 levels were performed again with increasing difficulty (Zebis et al., 2008) The main focus of the exercises were to improve awareness and neuromuscular control of 21

28 the hip, knee, and ankle muscles during standing, running, cutting, jumping, and landing tasks with simultaneous ball handling and included wobble board and balance mat exercises (Zebis et al., 2008). During this training, data on injury, match exposure, training frequency, and type of training were recorded for each week. Results of the test-restest analysis demonstrated high test-retest reproducibility for magnitude and timing of the neuromuscular activity, which indicates that the sidecutting maneuver is a consistent motor program that the athlete has developed over years of training (Zebis et al., 2008). Furthermore, the main finding of this study was that the neuromuscular training program induced a change in the pattern of neuromuscular activation of the medial hamstring muscles during sidecutting (Zebis et al., 2008). This selective increase in the semitendinosus activity in the pre-landing phase and initial landing phase, in parallel with unchanged neuromuscular activity of the quadriceps muscles, may represent an important adaptation in response to neuromuscular training. Dynamic valgus of the knee has previously been identified as a predisposing factor for ACL injury in female athletes (Besier et al., 2000; Hewett et al., 2005). Knowing this, the balance between medial-lateral hamstring recruitment appears to be highly important for the control of dynamic valgus at the knee. Female athletes have a disproportionately greater amount of EMG activity in their lateral hamstrings (biceps femoris) than male athletes when landing from a jump (Rozzi et al., 1999 as cited in Zebis et al., 2008); increased lateral hamstring motor unit firing potentially leads to a more open medial joint space and thereby potentially contributes to increased dynamic valgus. Another possible factor contributing to increased dynamic valgus was found by Myer et al. (2005), who added that female athletes tend to have a reduced medial-tolateral (VM-to-VL) quadriceps EMG ratio compared to male athletes. In order to counteract this dynamic valgus during sidecutting, it seems to be extremely important for the medial hamstring (semitendinosus/semimembranosus) to contract and compress the medial knee joint. The present study (Zebis et al., 2008) has established that the neuromuscular training program used, resulted in elevated preactivity of the medial hamstring and quadriceps muscles during sidecutting. The female athletes of the present study displayed a muscle activation pattern where the hamstring 22

29 muscles become activated before the quadriceps muscles, and before initial ground contact, both before and after training. A main finding of this study was that there was a selective change in EMG onset for the semitendinosus (medial hamstring) after training, which may reflect some sort of optimization of the motor program. In support of this optimization theory, other researchers (Cowling and Steele, 2001; as cited in Zebis et al., 2008) have found that during a deceleration task, the semimembranosus EMG onset occurred closer to the time of toe-down contact in males compared to females, which may have enabled hamstring muscle activity to better coincide with the high tibiofemoral sheer forces that are generated just after ground contact. These findings support the previous research that suggests that the medial hamstring plays a vital role in reducing the risk of noncontact ACL rupture in female athletes. This research also answers the question regarding whether or not neuromuscular training effects the activation of the medial hamstring. Furthermore, such information allows us to recognize a specific activation pattern in female athletes that may make them more or less susceptible to injury. In order to put their previous conclusions to the test, Zebis et al. performed a subsequent study in 2009.The objective of this study was to investigate the efficacy of a neuromuscular screening technique that may effectively predict ones risk of future ACL rupture in currently non-injured athletes. Zebis et al. (2009) hypothesized that currently non-injured female athletes with low preactivity of their knee flexor muscles, and high preactivity of their knee extensor muscles, during sidecutting maneuvers, would be at increased risk of future injury. Zebis et al. (2008) defined the sidecutting maneuver as a movement that the athlete is able to perform in match situations, when time for decision-making about postural correction is extremely limited. The purpose of the sidecutting maneuver is to fake the defensive player in one direction and then move in the opposite direction. In their previous investigation, Zebis et al. (2008) demonstrated high test-retest reproducibility for both the magnitude and timing of the EMG activity during sidecutting; this level of reproducibility indicates that the sidecutting maneuver is executed by a consistent motor program and has been found to remain unchanged during a regular season, even with in-season training and match play. 23

30 Prior to the start of their upcoming competitive season, 55 elite female team handball and soccer players (age, 24 ± 5 years; height, 169 ± 6 cm; weight, 69 ± 7 kg) were screened for both the pattern and magnitude of neuromuscular preactivity in the relevant leg muscles during the performance of a standardized sidecutting maneuver. EMG electrodes were placed on the medial portion of the vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) muscles of the quadriceps femoris muscle, and the biceps femoris caput longus (BFcl) and semitendinosus (ST) muscles of the hamstring muscle groups respectively. The subjects were then asked to perform five trials of the standardized sidecutting maneuver at a fixed distance of two meters to a force plate. In order to better simulate a game situation, the subjects were instructed to perform the maneuver as fast and a forceful as possible. The incidence of ACL ruptures were then recorded during the following two competitive seasons. During the two seasons following the neuromuscular screening, 5 athletes suffered an ACL rupture. A statistical analysis revealed that these injured athletes showed lower preactivity of the ST (21% ± 6% vs 40% ± 17%; P <.001) and higher preactivity of the VL (69% ± 12% vs 35% ± 15%; P <.01) compared with the uninjured players, while no significant differences were found for any of the other muscles examined. The difference between VL and ST EMG preactivity (ΔVL ST) was 47% ± 14% for the subsequently injured athletes and 2% ± 25% for the uninjured (P =.0006). Based on these findings, Zebis et al. (2009) defined a high-risk zone as one standard deviation above the mean VL-ST difference. It is also important to note that 10 of the 55 athletes screened in this study fell into the high-risk zone; of these 10 athletes, 5 were subsequently injured. The results of Zebis et al. (2009) are highly relevant for preventative sports medicine; if future studies are able to confirm this screening technique, as well as establish a successful method for correcting this neuromuscular deficit seen in female athletes, it is possible that we may be able to more effectively reduce the injury rate. The current review of the literature has largely focused on the identification of biomechanical mechanisms of injury and gender differences that may predispose female athletes to an increased injury-risk. In addition, prevention methods used to reduce the unacceptably high number of ACL ruptures that occur in female sports, which involve repetitive jumping and cutting, have 24

31 also been addressed. Although it has been well established that most noncontact ACL injuries occur in sporting situations that involve landing, sidecutting, and deceleration, the pivotal issue has been the identification of critical biomechanical and physiological risk factors. Studies such as Colby et al. (2000) and Besier et al. (2001a,b) have found that the sporting maneuvers typically associated with ACL injury involve substantial eccentric muscle force of the knee extensors and consequently, a substantial amount of anterior-directed shear force on the tibia relative to the femur occurs. This shear force on the tibia places a great deal of stress on the ACL, making it more susceptible to injury, and so it seems essential that this shear force needs to be counteracted through the appropriate co-activation of the knee flexor muscles (hamstrings). As several studies have established, during a sidecutting task, injuries occur when the knee is near full extension and in valgus combined with internal/external rotation of the tibia (Colby et al., 2000; Besier et al., 2001a,b; Besier et al., 2003). Thus, appropriate activation of the medial knee flexor muscles become extremely important in compressing the medial knee joint and resisting the valgus moment. More recent studies have been able to conclude that the sidecutting maneuver is a consistent motor program that the athlete develops over years of training (Zebis et al., 2008). Furthermore, it has also be found that the implementation of prophylactic neuromuscular training is effective in changing the pattern of neuromuscular activation of the hamstring muscles during sidecutting and that the resultant selective increase in semitendinosus activity during the pre-landing and initial landing phase, in parallel with unchanged neuromuscular activity of the quadriceps muscles, may represent an important adaptation in response to neuromuscular training. Based on such findings, it has been proposed that an increased ratio between the medial and lateral hamstrings neuromuscular activity may help to prevent excessive external rotation of the tibia and lateral knee joint compression during the sidecutting maneuver, thereby decreasing the risk of dynamic valgus (Zebis et al., 2008). Evidence from Zebis et al. (2009) then provided support for this theory by developing a neuromuscular screening method as a way to document a specific mechanism of future injury by identifying a specific neuromuscular activation pattern that appears to be predisposing for future ACL rupture. 25

32 While it has been demonstrated that the implementation of prophylactic neuromuscular training programs are effective in reducing the incidence of non-contact ACL injury in female athletes, evidence supporting the effectiveness of specific elements of such programs, as well as the underlying neuromuscular adaptation mechanisms elicited by this type of training is extremely limited. To date, there has only been one investigation that has identified the specific neuromuscular adaptations that are elicited by one such training program (Zebis et al., 2008). To complicate matters, a single standardized intervention program has not yet been established. This leads to the implementation of a number of different training programs throughout the world, making it rather difficult for one to conclude that the adaptation mechanisms elicited by one training program would be similar to the adaptations elicited by another. Most of training programs that have been proven to be effective are typically 6-8 weeks in duration, even though there is a lack of research to determine the ideal duration of these programs (Alentorn-Geli et al., 2009). In addition, prevention programs that are oriented only toward reducing non-contact ACL injury rates in female athletes have demonstrated lower athlete compliance rates than programs that also incorporate training components that target measures of improved athletic performance (Alentorn-Geli et al., 2009). Orientation and duration are two of the main drawbacks regarding the neuromuscular training program previously investigated by Zebis et al. (2008). Attempting to implement a 12-month program tends to interfere with a coach s regular in-season training and also makes it difficult for athletes to adhere a program of this length. Therefore, the purpose of this study was to investigate the neuromuscular adaptations elicited by a six-week prophylactic neuromuscular training program on knee joint motor control, during sidecutting, in high school female athletes. Based on the findings of previous research, it was hypothesized that prophylactic neuromuscular training would significantly alter the neuromuscular activation of the lower extremity musculature in a way that potentially reduces specific risk factors that predispose female athletes for future ACL rupture. 26

33 MATERIALS & METHODS Participants Sixteen competitive female soccer (5), basketball (8), and field hockey (3) players (age = ± 1.13 yr; height = ± 8.7 cm; mass = ± 8.1 kg; BMI 20.2 ± 1.63) from high schools in the greater Cincinnati area volunteered to participate in the current study. This study was approved by The Jewish Hospital of Cincinnati (Cincinnati, OH) Internal Review Board for the use of human subjects prior to participant recruitment. Of the sixteen participants, ten had never participated in a formal neuromuscular training program, whereas six had previously participated in the neuromuscular intervention program used in the current study 1-year prior. All participants had no history of knee injury or pathology and had no symptoms of pain, patella instability, or visible joint effusion at the time of recruitment. All testing and training procedures were fully explained, and written informed parental consent (appendix A) was obtained for each participant prior to the start of the investigation. Procedures After obtaining informed consent, measurements of height, weight, and body mass index (BMI) were recorded for each participant. Leg dominance was determined for all participants by establishing which leg the participant preferred to use when performing a single leg hop for distance (Chappell, Yu, Kirkendall, & Garrett, 2002). All participants were tested for neuromuscular activity at the knee joint during sidecutting, both before and after the implementation of the 6-week prophylactic neuromuscular training program. Prior to testing, all participants were familiarized with the testing procedures. Participants then performed a dynamic warm-up (appendix B) for each of the major muscle groups to be used during data collection. After completing the dynamic warm-up, a visual demonstration of the standardized sidecutting maneuver was performed by the investigator in order to familiarize the participants with the movement. Following the visual demonstration, participants were allowed to practice the sidecutting maneuver until they felt comfortable. Once comfortable with the maneuver, each participant performed 5-recorded trials of the sidecutting maneuver on both their dominant and non-dominant leg. Following each trial, participants were provided with one 27

34 minute of rest in order to minimize any effects of muscular fatigue. Any participants who failed to complete at least 16 of the 18 training sessions were excluded from the study. Sidecutting Maneuver. The sidecutting maneuver is a movement that the player is able to perform in match situations when time for decision-making about posture correction is extremely limited. The purpose of the sidecutting maneuver is for the offensive player to fake the defensive player in one direction and then quickly move in the opposite direction. A previous study has demonstrated high test-retest reproducibility for the magnitude and timing of EMG activity during sidecutting, indicating that this maneuver is executed by a consistent motor program, which has been found to remain unchanged during an athlete s regular season with training and match play (Zebis et al., 2008). The standardized sidecutting maneuver was performed with a fixed distance of 2 meters to a target area (Figure 1). Verbal instructions were given to the participants to perform the sidecutting maneuver as fast and as forceful as possible in order to best simulate a match situation. Prophylactic Neuromuscular Training Program. The prophylactic neuromuscular training program (Sportsmetrics TM ) used in this study was developed at the Cincinnati Sports Medicine Research and Education Foundation located in Cincinnati, Ohio ( Sportsmetrics TM is a 5-component program that meets 3 times per week over a 6-week period (18 sessions). The 5 components of Sportsmetrics TM include a dynamic warmup, jump training (always performed first), speed and agility training, strength training, and flexibility. The training sessions lasted approximately minutes and were held on alternating days (i.e. Monday, Wednesday, and Friday). During each session, participants were trained on safe jumping and landing techniques, jumping for increased vertical height, and increased muscular strength along with other activities aimed to increase athletic performance. Three phases were implemented throughout the jump-training program (Table 1). The technique phase (Phase I) included the initial 2 weeks, when proper jump technique was demonstrated and drilled. Four basic techniques were stressed: 1) correct posture (i.e., spine erect, shoulders back) and body alignment (e.g., chest over knees) throughout the jump; 2) jumping straight up without 28

35 any excessive side-to-side or forward-backward movement; 3) soft landings including toe-to-heal rocking and bent knees; and 4) instant recoil preparation for the next jump. Phrases such as on your toes, straight as an arrow, light as a feather, shock absorber, and recoil like a spring were used as verbal (and visualization) queues for each phase of the jump. The fundamentals phase (Phase II) concentrated on the use of proper technique to build a base of strength, power, and agility. Finally, the performance phase (Phase III) focused on achieving maximal vertical jump height. Throughout each session of the first two phases, exercises were increased in duration. Each athlete was encouraged to do as many jumps as possible, using the proper technique, in the time allotted. As the athletes became fatigued, they were encouraged to stop if they could not execute each jump correctly. During Phase III, athletes concentrated on the height achieved on each jump and the quality of each jump. Thirty seconds of recovery time were allotted between each exercise. Definitions of each exercise employed in the program are detailed in Table 2. Instrumentation Surface electromyography (S-EMG) was used in order to quantify muscle activation activity of the musculature surrounding the knee joint, using an 8 channel telemetered EMG system (Pocket EMG, BTS Bioengineering, Milan, Italy). The electrode sites were shaved with a hand razor and carefully cleaned with ethanol prior to electrode placement on both the dominant and nondominant leg. In an effort to ensure precise and standardized placement of the electrodes at each visit, recommendations developed by SENIAM (Surface ElectroMyoGraphy for the Noninvasive Assessment of Muscles) were carefully followed (appendix C). Bipolar Ag/AgCl surface EMG electrodes (Kendall ARBO H124SG, Gosport Hampshire, UK), with a 10 mm interelectrode distance, were placed on the medial portion of the vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) muscles of the quadriceps femoris muscle group. In addition, electrodes were also placed on the biceps femoris caput longus (BFcl) and semitendinosus (ST) muscles, representing the lateral and medial hamstring muscle groups, respectfully. Medical tape was then applied in order to ensure electrode placement, minimize motion artifact, and provide strain relief for the electrode cables. All data recordings were streamed continuously to a Dell notebook computer, where the data were then saved for later 29

36 analysis. The timing of initial ground contact during the sidecutting maneuver was synchronized with the EMG system using footswitches (BTS Bioengineering, Milan, Italy) that conveyed the data in real-time. EMG Signal Treatment During off-line analysis, all EMG signals were high-pass filtered at 20-Hz cutoff frequency (Butterworth filter) and subsequently smoothed by a symmetrical moving root mean square (RMS) filter of 30 milliseconds. The RMS EMG activity (mean average amplitude) was obtained in predefined time intervals and subsequently normalized to the peak RMS EMG amplitude recorded during the sidecutting maneuver. This procedure of EMG normalization has been found to have a higher reproducibility for the musculature examined during sidecutting compared to a procedure that normalizes data to the peak EMG amplitude obtained during a maximal voluntary contraction (MVC) (Zebis et al., 2008). Neuromuscular activity refers to the magnitude of the normalized RMS EMG amplitude in the given time intervals examined. Neuromuscular activity during the sidecutting maneuver was obtained at two time intervals (10- and 50-ms) before initial ground contact with the target area (prelanding phase) and again at two time intervals (10- and 50-ms) after initial ground contact with the target area (landing phase). Data were then subsequently normalized to the peak RMS EMG amplitude recorded during the sidecutting maneuver. A co-contraction ratio (CCR) was determined in order to define the relative activation of the flexor and extensor muscles crossing the knee. The CCR was calculated by first obtaining the normalized values for both the quadriceps and hamstring muscle groups during each targeted time-point. Afterwards, the normalized values were summed to represent the quadriceps and hamstring muscle groups in their entirety. The hamstring value was used as the divisor if it was found to be greater than the quadriceps value; however, the quadriceps value was used as the divisor if it was greater than the hamstrings value. This method of calculation resulted in the cocontraction ratio value always being less than or equal to 1. A co-contraction ratio closer to 1 would indicate excellent co-contraction, whereas values closer to 0 would represent poor cocontraction between the quadriceps and hamstring muscle groups. This ratio represents a 30

37 component of joint stability that allows for the relative activation of the knee flexor and extensor muscle groups crossing the knee joint (Besier et al., 2003; Lloyd & Buchanan, 2001). In addition, a medial-to-lateral hamstring activation ratio was calculated in order to evaluate hamstring muscle balance. Normalization to Peak Amplitude Obtained During Sidecutting For each sidecutting maneuver trial, the average RMS EMG amplitude in the predefined time intervals was normalized to the peak amplitude measured in the same trial. This normalization procedure was done for each muscle examined (Example: Trial 1, semitendinosus) Neuromuscular activity i = where i denotes the time interval. RMS EMG amplitude i Peak EMG amplitude TRIAL 1 Using this procedure, the neuromuscular activity at a given time interval for each participant as the average of 3 trials performed at each testing session (i.e., before and after prophylactic training). It should be noted that although 5 trials were recorded for both the dominant and nondominant leg before and after training, only 3 trials were averaged because of signal artifact found during off-line analysis. This also accounts for the disagreement in sample size between the dominant and non-dominant leg. Statistical Analyses Differences between pre- and post-training neuromuscular activation values for each muscle were assessed using the non-parametric Wilcoxon signed rank test for repeated measures. In addition, mixed-model analyses of variance were performed using leg-dominance/training-status as the fixed effect and subject as the random effect. The response variables were average RMS EMG amplitudes for each muscle analyzed, co-contraction ratios (CCR), and medial-to-lateral hamstring activation ratios. A α level of 0.05 (5%) was accepted as statistically significant and all values are presented as mean ± SD. RESULTS Results revealed no significant interaction between training-status and neuromuscular activation strategies. Thus, neuromuscular activation between first-time participants and those who had 31

38 undergone Sportsmetrics training one-year prior where not different from one another; therefore, all participants were treated as a single sample for the analysis of pre- to post-training differences in neuromuscular activation. When assessing the effect of leg dominance on training effect (postpre differences), the Mixed Procedure did reveal a significant difference F (1, 8) = 6.88 (P = 0.03) for the biceps femoris (BFcl) muscle of the dominant leg at 10 ms post-landing. Results from the Wilcoxon signed rank test revealed a significant difference at the 10-ms postlanding time-interval for the dominant leg. Neuromuscular activity increased in the biceps femoris (BFcl) muscle from 28 ± 11% to 40 ± 15% (P < 0.01). In addition, the medial-to-lateral hamstring ratio significantly decreased from ± to ± (P = 0.015) at 10-ms post-landing for the dominant leg (Table 6). No other statistically significant differences were found for any other muscle at any other phase of the sidecutting maneuver. Descriptive statistics, along with the results of the Wilcoxon signed rank test, for both the dominant and non-dominant leg are detailed in Table 3 and Table 4, respectively. DISCUSSION The main finding of the present investigation is that the 6-week prophylactic neuromuscular training program was able to induce a change in the neuromuscular activation of the hamstring muscles during sidecutting. The selective increase biceps femoris EMG activity (28 ± 11% to 40 ± 15%) at the 10-ms time interval after initial ground contact (IC), in parallel with the unchanged neuromuscular activity of the quadriceps muscles, may represent an important adaptation in response to neuromuscular training. High-risk maneuvers, such as sidecutting, involve substantial eccentric muscle force of the knee extensors during deceleration; consequently, a significant amount of anterior force is placed on the tibia, which results in increased stress on the ACL (Besier et al., 2001a,b; Colby et al., 2000; Simonsen et al., 1999). In addition, the movement of a limb segment typically involves some degree of agonist and antagonist cocontraction in order to maintain joint stability (Draganich et al., 1989). Therefore, the observed increase in neuromuscular activation of the biceps femoris muscle may serve to help decrease anterior shear forces on the tibia, thereby reducing the load placed on the ACL in the sagittal plane. 32

39 There is currently an ongoing debate in the literature regarding whether sagittal or non-sagittal factors play a more dominant role as a primary mechanism for non-contact ACL injury in female athletes. A cadaveric study demonstrated that aggressive quadriceps loading (4500 N) alone could result in ACL rupture (DeMorat, Weinhold, Blackburn, Chudik, & Garrett, 2004). However, this claim has been disputed by McLean et al. (2004, 2005) who argued that loading in the sagittal plane alone could not produce such injuries. In a prospective cohort study among female athletes, Hewett et al. (2005) showed that women who tore their ACL in-season demonstrated knee valgus moments 2.5 times that of those athletes who did not suffer from ACL injury, suggesting that valgus loading is also an important component. In support of this theory, video analyses of non-contact ACL injuries have also shown that valgus collapse appears to the main mechanism among female athletes (Krosshaug et al., 2007; Olsen et al., 2005). Furthermore, simulation studies have reported that valgus loading would substantially increase ACL force in situations where anterior tibial shear force is applied (Besier et al, 2001a,b; Markolf et al., 1995). The noted increases in knee abduction motion and moments displayed by females who have subsequently gone on to rupture their ACL might imply decreased neuromuscular control in the coronal plane (Hewett et al., 2005). Therefore, balance between the medial and lateral hamstring recruitment may play a very important role in limiting dynamic valgus and reducing anterior shear force on the tibia. Neuromuscular training programs have demonstrated significant positive alterations in movement biomechanics, lower extremity strength, and recruitment (Hewett et al., 1996). Additionally, prospective studies have demonstrated that such training programs have the potential to reduce ACL injury rates in female athletes (Barber-Westin & Noyes, 2009; Hewett et al., 1999; Myklebust et al., 2003). However, despite the success of such programs, studies identifying the underlying neuromuscular adaptation mechanisms elicited by this type of training are limited. To the best of our knowledge, there has only been one other investigation that has analyzed the underlying neuromuscular adaptation mechanisms elicited by prophylactic training (Zebis et al., 2008). Zebis et al. (2008) demonstrated that 12-months of neuromuscular training resulted in a significant increase in neuromuscular activation of the semitendinosus muscle at the 10-ms time interval both before and after IC, while quadriceps muscle activity remained unchanged. From these findings, it was concluded that the selective increase in activation of the 33

40 medial hamstrings might act to compress the medial joint space, thereby reducing the risk of dynamic valgus and future injury to the ACL (Zebis et al., 2008). In support of this theory, uninjured female athletes that display reduced EMG pre-activity of the semitendinosus muscle and increased pre-activity of the vastus lateralis muscle during sidecutting have been found to be at increased risk of future non-contact ACL rupture, emphasizing the importance of the medial hamstring (Zebis et al., 2009). It has also been suggested that an increased ratio between the medial and lateral hamstring neuromuscular activity may help to prevent excessive external rotation of the tibia and lateral joint compression during sidecutting maneuvers (Zebis et al., 2008). It should be noted that the 6-week training program used in the current study resulted in a selective upregulation of the lateral hamstring, which is in direct opposition of the findings presented by Zebis et al. (2008). Thus, further discussion is warranted in order to interpret this discrepancy. Although dynamic valgus collapse has been reported to be an important component in noncontact ACL injury among female athletes (Hewett et al., 2005; Krosshaug et al., 2007; Olsen et al., 2005), cadaver studies and mathematical simulations have argued that pure valgus motion would not produce ACL injuries without tearing the medial collateral ligament (MCL) first (Bendjaballah, Shirazi-Adl, & Zukor, 1997; Mazzocca, Nissen, Geary, & Adams, 2003; Shin, Chaudhari, & Andriacchi, 2009). In an investigation of MRI s from both contact and non-contact ACL injuries, Fayad et al. (2002) reported that a complete rupture of the MCL was present in only 6% of the cases studied. While this suggests that valgus loading alone is unlikely to result in ACL rupture, others have reported significant increases in ACL stress when valgus loads are combined with anterior tibial shear (Markolf et al., 1995). Therefore, if combined loading states in the sagittal and coronal planes were the only risk factors that predisposed female athletes for injury, increased co-activation of the hamstring muscles could potentially act to reduce dynamic valgus and resist the anterior tibial shear caused by quadriceps contraction. However, internal or external rotation of the tibia relative to the femur has also been reported to occur at the time of injury, suggesting that the primary mechanism of non-contact injury is likely 3-dimensional. 34

41 Valgus loading has been shown to induce a coupled motion of valgus and internal rotation of the tibia in cadaver models (Matsumoto et al., 2001). In support, Speer et al. (1992) reported that bone bruises of the lateral femoral condyle or posterolateral portion of the tibial plateau occurred in more than 80% of acute non-contact ACL injuries, which suggests that valgus in combination with internal rotation and/or anterior tibial translation occurred at the time of injury. Markolf et al. (1995) reported that the addition of internal tibial torque to a to a knee already loaded by anterior tibial force resulted in the greatest ACL forces and should be considered the most dangerous in terms of potential injury to the ACL. In contrast, video analyses have suggested that valgus in combination with external rotation is the most frequent motion pattern (Olsen et al., 2004). Until recently, video analyses of injury tapes have been the only method available to extract kinematic data from actual injury situations and have been largely limited to simple visual inspection (Boden et al., 2000; Cochrne et al., 2007; Olsen et al., 2004). New software advances have led to a model-based image-matching (MBIM) technique, which allows researchers to extract joint kinematics from video recordings using 1 or more uncalibrated cameras and has been found to be feasible for use in actual ACL injury situations (Krosshaug & Bahr, 2005; Krosshaug et al., 2007). Using MBIM technology, a more recent study reported that rapid knee valgus develops within 40-ms after IC, supporting the theory that valgus loading is a key factor in the ACL injury mechanism (Koga et al., 2010). At the same time, the tibia rotated internally, confirming that knee valgus and internal tibial rotation is a coupled motion (Koga et al., 2010). These findings allowed Koga et al. (2010) to develop a more robust hypothesis for the mechanism of non-contact ACL injury. Koga et al. (2010) proposed that (1) when valgus loading is applied, the MCL becomes taut and lateral joint compression occurs. (2) This compressive load, as well as the anterior tibial shear force caused by quadriceps contraction, causes a displacement of the femur relative to the tibia where the lateral femoral condyle shifts posteriorly and the tibia translates anteriorly and rotates internally, resulting in ACL rupture (Koga et al., 2010). (3) After the ACL is torn, the primary restraint to anterior translation of the tibia is gone. This causes the medial femoral condyle to also be displaced posteriorly, resulting in external rotation of the tibia (Koga et al., 2010). 35

42 If future kinematic studies are able to validate this 3-dimensional mechanism of injury, such findings may elucidate the critical role of appropriate hamstring activation in order to reduce dynamic knee valgus, counteract the anterior tibial shear produced by quadriceps contraction, and resist internal rotation of the tibia. Increased hamstring recruitment may provide dynamic knee stability by resisting anterior and lateral tibial translation, as well as transverse tibial rotations. Evidence has shown that the medial and lateral hamstrings are selectively activated to control internal and external rotations of the tibia, and EMG activity of the lateral hamstrings during the pre-landing and initial landing phase of sidecutting maneuvers may be a critical factor for preventing such rotation (Besier et al., 2003). Therefore, the selective upregulation of the lateral hamstring (biceps femoris) found in the current study may reflect a re-programming of the athletes existing motor program in order to more effectively counteract the anterior shear forces placed on the tibia, as well as limit the internal tibial rotation that has been reported to occur during sidecutting tasks. Notably, the medial-to-lateral hamstring ratio significantly decreased from ± to ± (P = 0.015) 10-ms after IC following training (Table 6; Figure 2). This finding suggests that as subjects learned how to jump, land, and cut more effectively during the prophylactic training program, they might have also learned to maximize the effect of the lateral hamstrings, thereby protecting the ACL. Based on the 3-dimensional mechanism of injury proposed by Koga et al. (2010), the adaptations elicited by the current intervention appear to be favorable for reducing non-contact injury risk. While these findings differ from those previously reported by Zebis et al. (2008), caution should be taken when interpreting these results. Because non-contact injury appears to be due to a combination of loading in all three planes of motion, it is likely that increased activation of the medial hamstring also reduces injury risk by resisting anterior shear force on the tibia and limiting dynamic valgus. Therefore, reducing the moments in one or more planes may be enough to decrease overall injury risk. However, additional research is needed in order to determine whether increased activation of the medial hamstring results in unfavorable internal rotation of the tibia, which could actually increase the amount of stress on the ACL. It should also be 36

43 pointed out that these two studies differed with respect to program duration (12 months vs. 6 weeks), age of the subjects (26 ± 3 vs. 15 ± 1 years) and level of competition (Elite vs. High School). Thus, the athletes studied previously are likely to have more athletic experience. Athletic experience has been thought to effect the development of at-risk performance patterns for ACL injury. While it was once thought that less experienced athletes were at increased risk for injury (McLean et al., 1999), more recent investigations reported that these athletes display significantly smaller flexor, adductor, and internal rotator peak moments and smaller net joint moment impulse in all three planes of motion at the knee compared to female athletes with more experience (Sigward & Powers, 2006). In addition, novice athletes have been reported to display greater co-contraction at the knee, which suggests that these athletes may adopt a more protective movement strategy in response to a relatively less-familiar task (Sigward & Powers, 2006). It has been suggested that with experience, a pattern of reduced co-contraction and greater knee moments appears to emerge (Sigward & Powers, 2006). This is consistent with the principles of skill acquisition, where the initial response to a relatively novel task (i.e., mass cocontraction) is progressively refined by the repression of unnecessary muscular activity into a more efficient movement strategy (Schneider et al., 1989; Young & Marteniuk, 1995; Humphrey & Reed, 1983). These findings imply that athletes may become more susceptible to injury as they gain more experience and confidence with this task. Therefore, fundamental differences in overall movement strategy between the athletes in the current investigation and those studied previously might contribute to the conflicting results. A secondary finding of the current investigation was that those athletes who had previously undergone the Sportsmetrics training program, one-year prior to this study, did not display differences in neuromuscular activation characteristics when compared to those who had not previously undergone training. This finding suggests a lack of retention from previous training. Myklebust et al. (2003) demonstrated that participation in an injury prevention program 3-times per week over a 5- to 7-week training period during the preseason and then once a week during the competitive season resulted in decreased injury rates for female athletes. However, it was found that injury rates returned to pre-training levels within 1-year after teams had discontinued their injury prevention programs (Mykelbust & Bahr, 2005). With the main goal of most injury 37

44 prevention programs being to increase movement quality in order to decrease injury risk, reports of elevated injury rates following the cessation of injury prevention training has caused some to question the ability of these programs to facilitate the retention of such changes (Pauda et al., 2012). A recent study, which compared the retention of improvements in movement patterns between a short-duration (3 months) and extended-duration (9 months) injury prevention program, demonstrated that individuals who completed the extended-duration program successfully retained changes in overall movement quality following a 3-month detraining period (Pauda et al., 2012). While both groups showed significant improvements from pre- to post-training, individuals who completed the short-duration program regressed back to their pre-training levels of movement quality following the detraining period (Pauda et al., 2012). As motor learning theory suggests, learning a new skill (eg, movement pattern) should be accompanied by relatively permanent changes in the performance of the task (Wulf, Shea, & Lewthwaite, 2010). Although both groups displayed significant improvements pre- to post-training, the finding that only the extended-duration program was able to elicit a maintained improvement following the completion of the program suggests that extended-duration training periods may be required in order to facilitate long-term retention of movement control (Pauda et al., 2012). The durations of neuromuscular training programs that have been proven to be effective in decreasing predetermined risk factors for ACL injury are typically 6-8 weeks. However, there is no clear consensus on the ideal duration for such programs (Alentorn-Geli et al., 2009). Although the 6-week training program used in the current study has been shown to reduce the incidence of non-contact ACL injury in female athletes (Hewett et al., 1999), the biomechanical and neuromuscular adaptations observed immediately after program completion does not necessarily appear translate to long-term retention. It has been argued that extended duration programs allow participants continued practice beyond the point of initial improvement, which may lead to the overlearning of safe movement patters and result in greater retention over time (Pauda et al., 2012). This suggests that training duration may be an important factor to consider when designing and implementing injury prevention programs. While previous research has examined the retention effects of program duration on movement quality, there have not been 38

45 any investigations that have assessed the retention of changes in neuromuscular activation. Therefore, additional investigations are needed in order to determine the effects of program duration on the retention of changes in neuromuscular activation in order to improve the effectiveness of current injury prevention programs and ensure that individuals develop permanent changes in neuromuscular control. LIMITATIONS The main limitation to the current investigation was the absence of a control or comparison group. Due to the demographics of the female athletes used in this study, gaining access to athletes that do not intend to participate in the prophylactic neuromuscular training program is a difficult task. It has previously been demonstrated that neuromuscular activity, onset of EMG, knee joint angles, and hip joint angles remain unchanged for the sidecutting maneuver during a 6-month control season, indicating that this maneuver is executed by a consistent motor program that is unaffected by regular in-season training and competition (Zebis et al., 2008). These findings suggests that any alterations in neuromuscular activity pre- to post-training are likely a result of the current intervention; however, such a claim cannot be made at the present time due to the lack of a control group. Although EMG is a widely used screening tool, there are certain limitations associated with this method. Given the inherent variability of EMG data, it is possible that the statistical power may have been limited due to the current sample size (N = 16). In addition, muscle activation measured during dynamic tasks is not the same as muscle force production. Although normalized EMG is a measure of muscle activation, which is an important determinant of muscle force and linearly correlated to muscle force, many other factors such as muscle cross-sectional area, muscle fiber length, and contractile velocity can also affect muscle force. Thus, the noted increases in hamstring muscle activation may not directly result in decreased ACL loading. Furthermore, EMG amplitude varies between individuals because of differences in skin conductance, thickness of subcutaneous fat, muscle fiber pennation angle, and so on. Nonetheless, the present method of normalizing EMG amplitude to the peak EMG during the 39

46 sidecutting maneuver has previously been shown to result in a highly reproducible EMG pattern (Zebis et al., 2008). Another potential limitation of this study was the inclusion 6 participants who had undergone Sportsmetrics training 12-months before the current investigation began. However, when compared to the 10 participants who had not participated in any prior training program, no significant differences were found. This information led to the interpretation that these athletes did not display a retention effect from previous training. We therefore felt that it was valid to assess the group as a whole in evaluating the neuromuscular adaptations elicited by the current training program. In addition, the 6-week training program elicited a significant change in neuromuscular activation of the dominant leg only. Due to a limited amount of research regarding limb dominance and neuromuscular activation, we are unable to explain these findings at the present time. Finally, it should be noted that a primary concern with lab-based assessments of sport movements has been the extent to which anticipated tasks accurately reflect the game environment. Besier et al. (2001b) have acknowledged this potential discrepancy, finding that the unanticipated execution of the sidecutting maneuver produced significant increases (~ 70%) in external varus/valgus and internal/external knee moments compared with movements that lessclosely reflected game play. In the current investigation, the participants were made aware of the task prior to execution, allowing the athlete to plan their movement strategy in advance. When destabilizing forces are anticipated, the CNS is capable of adjusting muscle activation patterns to oppose these forces. In contrast, athletes have been reported to use a more generalized cocontraction strategy during unanticipated conditions (Besier et al., 2003). During unanticipated conditions, muscle activation levels have been shown to only increase by 10-20%, resulting is a mismatch between muscle activation and the increased external loads applied to the joint (Besier et al., 2003). Currently, there are not any investigations that have examined the effects of prophylactic training on neuromuscular activation strategies during unanticipated sidecutting tasks. Therefore, we recommend that both anticipated and unanticipated conditions should be included in future research. 40

47 CONCLUSION The results of this study suggest that the neuromuscular characteristics of the lower extremity in female athletes can be improved through 6-weeks of prophylactic training. The current intervention selectively increased EMG activity in the lateral hamstring muscles, while the activation of the quadriceps and medial hamstring muscles remained unchanged. A decreased ratio between the medial (semitentinosus) and lateral (biceps femoris) hamstring muscles, in addition to increased co-activation of the musculature surrounding the knee joint, may help to reduce excessive internal rotation and anterior shear force on the tibia, thereby decreasing the amount of stress placed on the ACL. This observed neuromuscular adaptation during sidecutting could possibly decrease the risk for non-contact ACL injury among female athletes. 41

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56 Table 1. Jump Training Program Exercise Repetitions or time Phase I: Technique Week 1 Week 2 1. Wall jumps 20 sec 25 sec 2. Tuck jumps b 20 sec 25 sec 3. Broad jumps stick land 5 reps 10 reps 4. Squat jumps b 10 sec 15 sec 5. Double leg cone jumps b 30 sec/30 sec 30 sec/30 sec (side-toside and back-to-front) jumps 20 sec 25 sec 7. Bounding in place 20 sec 25 sec Phase II: Fundamentals Week 3 Week 4 1. Wall jumps 30 sec 30 sec 2. Tuck jumps b 30 sec 30 sec 3. Jump, jump, jump, vert. jump 5 reps 8 reps 4. Squat jumps b 20 sec 20 sec 5. Bounding for distance 1 run 2 runs 6. Double leg cone jumps b 30 sec/30 sec 30 sec/30 sec (side-toside and back-to-front) 7. Scissor jump 30 sec 30 sec 8. Hop, hop, stick b 5 reps/leg 5 reps/leg Phase III: Performance Week 5 Week 6 1. Wall jumps 30 sec 30 sec 2. Step, jump up, down, vertical 5 reps 10 reps 3. Mattress jumps 30 sec/30 sec 30 sec/30 sec (side-toside and back-to-front) 4. Single-legged jump distance b 5 reps/leg 5 reps/leg 5. Squat jumps b 25 sec 25 sec 6. Jump into bounding b 3 runs 4 runs 7. Single-legged hop, hop, stick 5 reps/leg 5 reps/leg a Before jumping exercises, athletes performed a dynamic stretching routine. After training, athletes performed an active cool down and flexibility training. Each jump was followed by a 30- second rest period. b These jumps were performed on mats. 50

57 Table 2. Glossary of Jump Training Exercises Jumps: Two-footed jump. Rotate 180 in midair. Hold landing for 2 seconds, then repeat in reverse direction. 2. Bounding for distance: Start bounding in place and slowly increase distance with each step, keeping knees high. 3. Bounding in place: Jump from one leg to the other straight up and down, progressively increasing rhythm and height. 4. Broad jumps-stick (hold) landing: Two-footed jump as far as possible. Hold the landing for 5 seconds. 5. Cone jumps: double leg jump with feet together. Jump side-to-side over cones quickly. Repeat forward and backward. 6. Hop, hop, stick: Single-legged hop. Stick second landing for 5 seconds. Increase distance of hop as technique improves. 7. Jump into bounding α : Two-footed broad jump. Land on single leg and then progress into bounding for distance. 8. Jump, jump, jump, vertical: Three broad jumps with a vertical jump immediately after landing the third broad jump. 9. Mattress jumps: Two-footed jump on mattress, tramp, or other easily compressed device. Perform side-to-side/back-to-front. 10. Scissors jump: start in stride position with one foot well in front of the other. Jump up, alternating foot positions in midair. 11. Single-legged jumps distance α : One-legged hop for distance. Hold landing (knees bent) for 5 seconds. 12. Squat jumps α : standing jump raising both arms overhead, land in squatting position touching both hands to the floor. 13. Step, jump up, down, vertical: Two-footed jump onto 6- to 8-inch step. Jump off step with two feet, then vertical jump. 14. Tuck jumps: from standing position, jump and bring both knees up to chest as high as possible. Repeat quickly. 15. Wall jumps (ankle bounces): with knees slightly bent and arms raised overhead, bounce up and down off toes. α These jumps are performed on mats. 51

58 TABLE 3. Dominant Leg: Neuromuscular Activation During Sidecutting Normalized to Peak Amplitude During Sidecutting (% of max) Pre-Training Post-Training Phase Muscle Mean ± SD Mean ± SD Wilcoxons S P Value 50 ms Pre-landing VL 32 ± ± VM 33 ± ± RF 23 ± ± BFcl 50 ± ± ST 58 ± ± ms Pre-landing VL 47 ± ± VM 50 ± ± RF 41 ± ± BFcl 36 ± ± ST 49 ± ± ms Post-landing VL 53 ± ± VM 53 ± ± RF 50 ± ± BFcl 28 ± ± * ST 33 ± ± ms Post-landing VL 64 ± ± VM 67 ± ± RF 66 ± ± BFcl 43 ± ± ST 38 ± ± * Statistically significant difference between pre- and post-training neuromuscular activation (P < 0.05). 52

59 TABLE 4. Non-Dominant Leg: Neuromuscular Activation During Sidecutting Normalized to Peak Amplitude During Sidecutting (% of max) Pre-Training Post-Training Phase Muscle Mean ± SD Mean ± SD Wilcoxons S P Value 50 ms Pre-landing VL 38 ± ± VM 35 ± ± RF 29 ± ± BFcl 50 ± ± ST 52 ± ± ms Pre-landing VL 54 ± ± VM 46 ± ± RF 47 ± ± BFcl 39 ± ± ST 37 ± ± ms Post-landing VL 60 ± ± VM 56 ± ± RF 57 ± ± BFcl 45 ± ± ST 36 ± ± ms Post-landing VL 68 ± ± VM 60 ± ± RF 66 ± ± BFcl 56 ± ± ST 50 ± ± * Statistically significant difference between pre- and post-training neuromuscular activation (P < 0.05). 53

60 TABLE 5. Co-Contraction Ratios Pre-Training Post-Training Phase Mean ± SD Mean ± SD Wilcoxons S P Value Dominant Leg 50 ms Pre-landing ± ± ms Pre-landing ± ± ms Post-landing ± ± ms Post-landing ± ± Non-Dominant Leg 50 ms Pre-landing ± ± ms Pre-landing ± ± ms Post-landing ± ± ms Post-landing ± ± * Statistically significant difference between pre- and post-training neuromuscular activation (P < 0.05). 54

61 TABLE 6. Medial-to-Lateral Hamstring Ratios Pre-Training Post-Training Phase Mean ± SD Mean ± SD Wilcoxons S P Value Dominant Leg 50 ms Pre-landing ± ± ms Pre-landing ± ± ms Post-landing ± ± * 50 ms Post-landing ± ± * Statistically significant difference between pre- and post-training neuromuscular activation (P < 0.05). 55

62 FIGURE 1. Experimental setup (not to scale). (A) Starting line, (B) 2 meter runway to target area, (C) target area, (D) high-speed video camera. 56