Alarge proportion of anterior cruciate ligament. Gender Differences in Frontal and Sagittal Plane Biomechanics during Drop Landings

Transcription

1 APPLIED SCIENCES Biodynamics Gender Differences in Frontal and Sagittal Plane Biomechanics during Drop Landings THOMAS W. KERNOZEK 1,2, MICHAEL R. TORRY 3, HEATHER VAN HOOF 1, HANNI COWLEY 1, and SUZANNE TANNER 2 1 Department of Health Professions, University of Wisconsin-La Crosse, La Crosse, WI; 2 Gundersen Lutheran Sports Medicine, La Crosse, WI; and 3 Biomechanics Research Laboratory, Steadman Hawkins Research Foundation, Vail, CO ABSTRACT KERNOZEK, T. W., M. R. TORRY, H. VAN HOOF, H. COWLEY, and S. TANNER. Gender Differences in Frontal and Sagittal Plane Biomechanics during Drop Landings. Med. Sci. Sports Exerc., Vol. 37, No. 6, pp , Purpose: To determine gender differences in lower-extremity joint kinematics and kinetics between age- and skill-matched recreational athletes. Methods: Inverse dynamic solutions estimated the lower-extremity flexion-extension and varus-valgus kinematics and kinetics for 15 females and 15 males performing a 60-cm drop landing. A mixed model, repeated measures analysis of variance (gender * joint) was performed on select kinematic and kinetic variables. Results: Peak hip and knee flexion and ankle dorsiflexion angles were greater in females in the sagittal plane (group effect, P 0.02). Females exhibited greater frontal plane motion (group * joint, P 0.02). Differences were attributed to greater peak knee valgus and peak ankle pronation angles (post hoc tests, P 0.00). Females exhibited a greater range of motion (ROM) in the sagittal plane (group main effect, P 0.02) and the frontal plane (group * joint, P 0.01). Differences were attributed to the greater knee varus-valgus ROM, ankle dorsiflexion, and pronation ROM (post hoc tests). Ground reaction forces were different between groups (group * direction, P 0.05). Females exhibited greater peak vertical and posterior (A/P) force than males (post hoc tests). Females exhibited different knee profiles (Group main effect, P 0.01). These differences were attributed to a reduced varus in females (post hoc tests). Conclusion: The majority of the differences in kinematic and kinetic variables between male and female recreational athletes during landing were observed in the frontal plane not in the sagittal plane. Specifically, females generated a smaller internal knee varus at the time of peak valgus knee angulation. Key Words: ACL, KNEE, INJURY, GENDER, KINETICS, BIOMECHANICS Alarge proportion of anterior cruciate ligament (ACL) ruptures occur as a result of noncontact injuries (38,39). This observation is particularly germane to the female athlete where the position of the body during deceleration type of movements, such as landing from a jump, cutting, or pivoting, has been documented as a significant performance risk factor for ACL injury (1,2,21,36,39). The biomechanical explanation often given for this injury is a more extended knee position at ground contact, which results in higher external ground reaction forces, as well as a greater resultant force vector between the Address for correspondence: Thomas W. Kernozek, Department of Health Professions, University of Wisconsin-La Crosse, La Crosse, WI 54601; Kernozek.thom@uwlax.edu. Submitted for publication March Accepted for publication February /05/ /0 MEDICINE & SCIENCE IN SPORTS & EXERCISE Copyright 2005 by the American College of Sports Medicine DOI: /01.mss b 1003 patellar tendon and the tibia coupled with a large eccentric quadriceps contraction (8,39). These biomechanical factors culminate in increased anterior translation of the tibia relative to the femur, which can mechanically strain the ACL (24,39). These performance mechanics are also supported by functional electromyographic (EMG) studies that have shown females employ a neuromuscular strategy, defined by significantly greater quadriceps activation and significantly less hamstring activation (6,23). Decker et al. (7) have shown females tend to contact the ground in a more erect sagittal plane position, they exhibit greater knee flexion profile with greater knee flexion angular velocities that may actually protect the ACL by dissipating forces over the larger range of motion; additionally, this pattern may also allow for peak quadriceps torque to occur at greater flexion angles ( 45 ) when the ACL is less vulnerable. This view is reasonably supported by Pflum et al. (30) and McLean et al. (25), who similarly reported sagittal plane kinematics and quadriceps muscular forces do not produce aberrant ACL loads during landing or side-step cutting. Instead, both research groups suggest coupled

2 movements in the sagittal, frontal, and possibly the transverse planes may contribute more to ACL injury than sagittal plane kinematics and kinetics alone. Moreover, these computational modeling studies are supported by traditional in vitro studies, which have shown ACL injury can occur with isolated varus and valgus knee rotation (3,15,31), and these loads can be more deleterious when varus-valgus knee torque and anterior tibial forces are coupled (24). Thus, it is evident that an understanding of the in vivo varus-valgus knee positions as well as the varus-valgus knee torques acting in conjunction with large knee extensor torques would be important in more fully understanding the potential for noncontact ACL injury during the landing motion. Despite the strong evidence from in vitro studies (3,15,24,31) and the theoretical investigations of Pflum et al. (30) and McLean et al. (25), there have been few in vivo studies of the lower extremity comparing the varus-valgus knee positions and torques between genders during jumpstop or drop landing motions (25,30). Ford et al. (10) reported males land with greater knee valgus range of motion and experience a greater peak knee valgus angle compared with females. This disparity, they theorized, may help explain the differences in the noncontact injury rates between male and female athletes. Unfortunately, the authors did not report the associated frontal plane kinetic features of the motion, or the timing of such events. Inclusion of these parameters could offer considerable insight into how loading discrepancies could alter the joint loading patterns that were occurring in these athletes. Chappell et al. (5) examined 3D kinematic and kinetic takeoff and landing patterns between males and females during three different jump-stop motions. They showed that the landing phase generates larger anterior knee shear forces compared with the takeoff phase. Furthermore, their results suggested that females exhibit greater proximal tibia anterior shear forces as well as larger knee extension and valgus knee s during the landing phase compared with males. To date, most gender performance studies have focused on the magnitudes of lower-extremity joint kinetics during the drop landing. Few studies have sought to determine temporal performance differences within these mechanical factors. The paucity of temporal data obscures the ability to link performance to injury, as both magnitude and timing of deleterious events during landings are important to consider in most injury scenarios. Previous literature has established that the landing motion is associated with notable anterior knee shear that are thought to be closely associated with large knee extensor s (5). Furthermore, this profile, when coupled with varus-valgus knee s, can exacerbate ACL loads (15,24,31). It is further apparent that the temporal occurrences of the kinematic and kinetic events have not been appositely reported to date. Thus, it is logical that a focused effort at understanding the external forces, kinematics, and kinetics as well as the temporal characteristics of these parameters be undertaken in an attempt to identify important performance factors that may predispose individuals to higher ACL loads during the landing motion. The purpose of this study was to quantify gender differences in the frontal and sagittal plane kinematic (hip, knee, and ankle joint angles) and kinetic (hip, knee, and ankle joint s) profiles as well as the temporal occurrences of the peak events of these parameters during drop landings in young, adult recreational athletes. It was hypothesized that males and females will demonstrate different hip, knee, and ankle joint kinematic and kinetic biomechanics, suggesting different mechanisms of lower-extremity control, particularly in the frontal plane. METHODS Fifteen female (mean age yr, mean height cm 8.62, mean weight kg 6.35) and 15 male (mean age yr, mean height cm 7.55, mean weight kg 8.23) recreational athletes from a university campus were recruited to participate in this study. Participants had no history of serious lowerextremity injury. All subjects signed a written consent form approved by the university that was in accordance with the NIH-mandated IRB guidelines. All subjects wore their own tight-fitting clothing for exercise and testing, and all wore the same shoe type (running shoe, model 629, New Balance, Boston, MA) to reduce the inherent variability caused by different shoe-sole absorption properties. After a 10-min stretching and walk/jog (at selfselected speeds) warm-up period on a motorized treadmill, the subjects practiced the landing task. The landing task consisted of dropping 60 cm from an adjustable hang bar suspended over the force plate. Pilot work suggested more reliable kinematic and kinetic data (intraclass correlation coefficients ranging from 0.79 to 0.93, depending on the variable assessed) could be obtained by using a hang bar (as opposed to stepping off a box (7)). With ground contact, one foot landed upon a force platform flush with the landing surface (Bertec Corp., Columbus, OH), and the other foot landed next to the force platform on the landing platform. Kinetic data from only the right extremity were collected for analysis. The force plate sampled the ground reaction forces at 1200 Hz. The point of application of the ground reaction forces (GRF) on the body was calculated as the center of pressure. The 3D kinematics of each trial were captured by securing 18 retroreflective spherical markers (diameter 25 mm) to the test limb of each subject in a standard Helen Hayes configuration at anatomical landmarks reported previously (19). These include: right and left anterior superior iliac spine (ASIS), sacrum, right and left thigh approximately 15 cm superior to the superior pole of the patella (at the estimated line of the knee joint center), right and left lateral femoral condyles, right and left lateral tibia (half way between ankle and knee), right and left tibial tuberosity, right and left lateral and medial malleoli, right and left posterior portion of the calcaneus (on shoe), right and left superior navicular of the foot, and the right and left web space between metatarsals 1 and 2. With this marker configuration, the X,Y,Z coordinate distances of the hip joint 1004 Official Journal of the American College of Sports Medicine

3 center from the ASIS were determined as a function of leg length and greater trochanter location (4). The knee center was assumed to be in line with the plane defined by the thigh marker, the hip joint center, and halfway between the femoral condyles. The ankle joint center was assumed to fall in the plane defined by the estimated knee center, the tibial tuberosity, and halfway between the two malleoli markers. All kinematic data were collected at 240 Hz using 6 JCL cameras positioned at 60º intervals around the performance area. The cameras and subsequent performance area were calibrated, yielding mean residual errors of mm over a volume of m. The marker coordinate data were analyzed using Orthotrak (Motion Analysis Corporation, Santa Rosa, CA) and custom Matlab programs (Mathworks Inc., Natick, MA). Based on a frequency content analysis of the digitized coordinate data, marker trajectories were filtered at 10 Hz using a fourth order Butterworth filter. Force platform data were filtered with the same technique using the same cutoff frequency. An initial standing neutral (erect) standing position of the subject in the anatomical position allowed the transformation matrix between the two local coordinates systems described above. Joint angular position, velocities, and accelerations were calculated from the filtered 3D marker coordinate data. In deriving the segment rotations, the assumption was made that flexion-extension was the first rotation, followed by abduction and internal external rotation, respectively. Using the neutral trial as a reference, zero degrees at the hip, knee, and ankle corresponds to an erect (neutral), standing posture with the trunk, thigh, and lower leg in a straight line, and the foot segment at a right angle to the leg. By this convention, the frontal plane varus and valgus kinematics were assigned positive and negative values, respectively. The joint s referred to in this paper are internal joint s, or s applied from all the structures within and crossing the joint. Values for each joint were calculated using the Orthotrak commercial software (Motion Analysis Corporation, version 5.02, Santa Rosa, CA) by combining the kinematic and force plate data with anthropometric data (9) using the inverse dynamics solution (19,20). By convention, hip and knee extensor and ankle plantar-flexor s were assigned positive values, and varus and valgus knee joint s were assigned positive and negative values, respectively. Thus, an external knee valgus would tend to produce a valgus knee rotation that would be resisted by an internal knee varus. All force values and all joint parameters were scaled to percent body mass (%body mass) and Newtonmeter per kilogram of body mass, respectively. The time series data sets were interpolated to 100 points during the impact phase (defined as the period from initial force platform contact to maximal knee flexion) for graphical purposes only. Timing measures (i.e., landing time, timing of peak ground reaction force, and peak s) were compared with an independent t-test between groups. Pearson correlation coefficients were calculated between timing of peak extensor and the timing of the peak varus. The mean of the eight trials for each subject were computed for hip, knee, and ankle contact position and range of motion (ROM) measured over the landing phase period, which was delineated from initial ground contact to maximum knee flexion. Gender comparisons between the dependent variables were conducted with a 2 3 (group * joint) mixed factor ANOVA. In addition, the maximum hip, knee and ankle extensor joint s occurring during the impact phase were analyzed. A 2 3 (group * direction) mixed factor ANOVA was used to compare between groups between peak vertical (VGRF), anterior posterior (A/PGRF), and mediolateral (M/LGRF) ground reaction forces. Alpha was set at 0.05 for all statistical tests. During post hoc tests, alpha was corrected for multiple comparisons using the Bonferroni procedure. RESULTS Landing style and joint kinematics. All subjects were visually observed to perform forefoot-to-rearfoot landings and the group mean landing phase times were not different between groups (male group, s; female group, s; P 0.66, power 0.50). Group means and standard deviations for hip, knee, and ankle joint positions at initial contact with the force platform and peak hip, knee, and ankle angles occurring during the landing phase are presented in Tables 1 and 2. Males and females exhibited similar hip joint positions at initial contact in both the sagittal (P 0.05, power for group effect 0.64 and group * joint effect 0.11) and frontal planes (P 0.05, power for group effect 0.23 and group * joint effect 0.54). However, females exhibited greater peak ankle dorsiflexion (power for sagittal plane group effect 0.64 and group * joint effect 0.10), peak foot pronation, and peak knee valgus angles (group effect, P 0.05, power for frontal plane group effects 1.0 and group * joint effect 1.0) compared with males (Table 2). Mean values ( 1 SD) for frontal and sagittal plane hip, knee, and ankle ROM are listed in Table 3 and graphically depicted in Figures 1 5. There were differences between males and females in both sagittal (group effect, P 0.05, power 0.58) and frontal plane ROM (group * joint interaction, P 0.05, power 1.0). Specifically, females exhibited greater ankle dorsiflexion and foot pronation ROM compared with males (post hoc tests; Table 3). Although no ROM differences were noted at the knee in the sagittal plane, females exhibited significantly greater knee varus- TABLE 1. Means and standard deviations of the joint kinematics between male and female participants at initial contact in degrees. Joint Variable Male Female P Ankle Ankle plantarflexion (5.82) (5.82) Maximum foot pronation 0.72 (3.15) 0.96 (4.04) (supination ( )/pronation ( )) Knee Flexion (4.41) (5.51) Varus ( )/valgus ( ) 1.01 (3.69) 3.72 (3.06) Hip Flexion (7.41) (7.41) Adduction ( )/abduction ( ) 8.31 (4.86) 6.75 (4.56) *** Significant difference in post hoc testing. GENDER DIFFERENCES IN LANDING MECHANICS Medicine & Science in Sports & Exercise 1005

4 TABLE 2. Means and standard deviations of the peak joint kinematics between male and female participants in degrees. Joint Variable Male Female P Ankle Maximum ankle dorsiflexion (7.61) (5.82) *** Maximum foot pronation 0.91 (6.73) (8.06) *** (supination ( )/pronation ( )) Knee Flexion (11.43) (13.42) Varus ( )/valgus ( ) 0.66 (6.90) (8.45) *** Hip Flexion 8.85 (5.31) 9.25 (5.16) Adduction ( )/abduction ( ) 8.85 (5.31) 9.25 (5.16) *** Significant difference in post hoc testing. valgus ROM in the frontal plane. No kinematic ROM differences were detected at the hip between genders. Ground reaction forces. Ground reaction forces were different between groups (group * direction, P 0.05, power 0.99). Females exhibited greater peak vertical force and greater posterior ground reaction force than males (post hoc tests, Table 4). No differences in medial/lateral GRF values were detected. Joint s. No differences were found in the sagittal plane peak hip, peak knee extensor (Fig. 6), or peak ankle plantarflexor s between genders (P 0.05, power for frontal plane group effects 0.54 and group * joint effect 0.73; Table 5). However, differences were noted in the frontal plane s (group * joint interaction, P 0.05, power 0.91). Specifically, the female group exhibited a significantly lower peak knee varus during the landing phase (Table 5; Fig. 7). No differences were noted in the time to the peak varus or valgus knee (P 0.22, power 0.23), or in the time to the peak knee extensor between genders (P 0.46, power 0.99). However, there was a moderate correlation between the time of peak knee extensor and the time of the peak valgus knee (r 0.55, P 0.002); and a negative correlation between the peak knee extensor and the time of the peak varus knee (r 0.45, P 0.013) across both genders. For females, the peak varus knee occurred at a frontal plane, knee valgus angle of 11.2 and a concomitant sagittal plane, knee flexion angle of For males, the peak varus knee occurred at a frontal plane, valgus knee angle of 3.2 and a concomitant sagittal plane, knee flexion angle of However, no statistical differences in these angles were detected between genders (all P 0.07). TABLE 3. Means and standard deviations of the range of motion (ROM) of joint kinematics between male and female participants in degrees. Joint Variable Male Female P Ankle Ankle dorsiflexion/plantarflexion (10.23) (7.80) *** Foot pronation/supination 4.21 (4.98) (7.25) *** Knee Flexion/extension (14.81) (11.21) Varus/valgus 7.08 (6.61) (9.00) *** Hip Flexion/extension (17.26) (15.24) Adduction/abduction 8.25 (7.25) (5.11) *** Significant difference in post hoc testing. DISCUSSION Several investigators have identified gender differences in kinematic and kinetic landing profiles in the sagittal plane ( 5 7,10,14,16,23,27,32). Few, however, have attempted to quantify gender performance differences at the level of joint torque profiles in both sagittal and frontal planes for the lower extremity during landings (5). The results presented herein support our hypothesis and suggest that gender differences in knee joint biomechanics exist and are more evident in the frontal plane than in the sagittal plane during drop landings. Females exhibited greater peak ankle dorsiflexion during the landing phase. This difference is in agreement with Decker et al. (7), who reported that females utilized greater ankle dorsiflexion than males to absorb energy and accomplish the landing task safely. Decker et al. (7) hypothesized that this ankle strategy is beneficial and may decrease ACL load by dissipating more energy at the ankle instead of propagating it up to the knee joint. The females in the current study also demonstrated greater peak foot pronation angles compared with males. In relation to previous drop landing literature, this finding has not previously been reported and was an unexpected result in the present study. However, a similar result has been observed by McLean et al. (25) for sidestepping, with females demonstrating significant increases in stance phase pronation compared with males. McLean et al. (25) suggested that the potential influence of foot morphological differences on the resultant accuracy of the talocrural axis definition may be one cause of this gender difference. Moreover, we propose this increased ankle ROM in the frontal plane may also be a method of absorbing energy of the land and reducing the overall load transmitted to the knee. Similar to Ford et al. (10), females in the present study demonstrated greater peak knee valgus angles compared with males during the first 30 50% of the landing phase. The ACL has been shown to undergo greater strain in valgus compared with similar amounts of varus angulation (3,15,24). Bendjaballah et al. (3) reported that the load on the ACL could be six times higher with as little as 5º (from neutral) of knee valgus. The discrepancies between males and females in knee valgus position observed herein potentially represent important frontal plane differences during the landing motion. This finding is in agreement with others who have reported increased knee valgus positions in females compared with males during landing (5,10). It is important to note that the present study examined a motion that is fundamentally and biomechanically different then those movements examined by Chappell et al. (5) and Ford et al. (10). However, the similar findings across these studies suggests that the increased knee valgus motion is apparent in females across a variety of sporting movements that includes landing from a jump, cutting, and sidestepping (5 7,13,21,26,27), and we hypothesize that a common mechanism of the noncontact ACL injury for females may exist across a wide range of movements. The practical merit of this observation is further supported by epidemiological 1006 Official Journal of the American College of Sports Medicine

5 FIGURE 1 Average time normalized curve SD from eight landing trials for hip flexion angle in degrees for male and female participants. and clinical annotations that cite aberrant knee kinematic movements in the frontal plane as key elements to the noncontact ACL injury mechanism (17,18). Although the framework of this paper assumes that the gender differences in knee valgus angle during the landing is a performance-based occurrence, it is noted that these findings may also be due to anatomical differences between genders. Females tend to possess a larger Q-angle (relationship of femoral axis relative to the tibial axis measured at the knee in the frontal plane), which is often associated with an increased knee valgus position. Although we did not measure subject-specific Q-angles and cannot directly assess this association, no relationship between Q-angle and ACL injury rates in female athletes have been found to date (11,28). FIGURE 2 Average time normalized curve SD for knee flexion angle (degrees) from eight landing trials for male and female participants. GENDER DIFFERENCES IN LANDING MECHANICS Medicine & Science in Sports & Exercise 1007

6 FIGURE 3 Average time normalized curve SD for ankle flexion angle (degrees) from eight landing trials for male and female participants (positive values plantar flexion, negative values dorsiflexion). Nonetheless, noted gender differences in femoral anteversion, tibial torsion, and foot pronation (36,39) suggest that future research is warranted in these areas, and that female anthropometrics may be a confounding factor that cannot be discounted when attempting to understand gender-specific, in vivo, performance-based research. Moreover, and as suggested by recent research (29), anatomical differences in conjunction with neuromuscular differences (decreased neural activation) may further confound studies seeking to identify the gender differences in various athletic movements. Although joint kinematic data alone is informative, the likelihood of a knee injury occurring is most likely predi- FIGURE 4 Average time normalized curve SD for knee abduction angle (degrees) from eight landing trials for male and female participants (positive values varus, negative values valgus) Official Journal of the American College of Sports Medicine

7 FIGURE 5 Average time normalized curve SD for ankle abduction angle (degrees) from eight landing trials for male and female participants (positive values foot supination, negative values foot pronation). TABLE 4. Means and standard deviations of the peak ground reaction force between males and female participants as a percentage body mass. Mean (SD) Mean (SD) Difference Variable Male Female P Vertical ground reaction force 3.51 (0.63) 4.71 (0.71) *** Anterior ( )/posterior ( ) 0.78 (0.15) 0.19 (0.18) *** ground reaction force Medial ( )/lateral ( ) ground reaction force 0.43 (0.11) 0.37 (0.10) *** Significant difference in post hoc testing. cated on both the alignment and position of the lower extremity as well as the magnitude and direction of the external joint s at any instant in time (24). The knee s calculated herein are consistent with those of Salci et al. (33), Decker et al. (7), and Madigan and Pidcoe (22). Larger varus or valgus frontal plane knee angles in combination with higher and respective external varus or valgus knee s would tend to increase the risk of injury. Correspondingly, and with regard to internal knee s, larger varus or valgus frontal plane knee angles in combination with lower and respective internal frontal plane knee s would tend to increase the risk of injury, because the lower internal s cannot support the increasing frontal plane angulation. In this study, females exhibited significantly lower peak internal knee varus s compared with the males who demonstrated 42% increase in the mean peak internal knee varus. Because a high internal knee varus would serve to resist valgus knee excursion, this finding suggests that females are at a greater knee valgus position (although not statistically different) at a time when they are apparently unable to generate the internal varus required to limit the valgus motion. Additionally, the timing of peak valgus knee angle, peak knee varus-valgus s, and peak knee extensor knee suggest coupling of these knee kinetics, which may further predispose individuals to a the noncontact ACL injury. In the present study, there were no differences in the timing of these events between genders. Yet, the females exhibited less internal varus knee torque at the same instant the knee was at its peak valgus position and exhibiting its peak knee extensor torque (r 0.55 between these factors). Moreover, these loads occur in a female knee that previous research has shown to demonstrate increased joint rotational laxity compared with males (37). Collectively, we interpret the timing relationships in the peak knee varus-valgus and the peak knee extension magnitudes to suggest that females are more prone to higher ACL loads during the drop landing because of the greater valgus knee position combined with a low varus at the same time that a high knee extensor is being applied to the joint. We acknowledge that this interpretation requires further study, as we do not know if the larger knee valgus excursion observed in the females is indeed caused by lack of internal varus knee, anatomical variations, neuromuscular differences, or some combination of these or other unidentified factors. The magnitude and timing of the VGRF for both genders were similar to results of previous studies (7,40). However, in contrast to a previous report (7), the females in the present study exhibited a larger peak VGRF. This was unanticipated because no differences were measured in sagittal-plane hip and knee flexion angles or in hip and knee extensor s between genders, and because the females experi- GENDER DIFFERENCES IN LANDING MECHANICS Medicine & Science in Sports & Exercise 1009

8 FIGURE 6 Average time normalized curve SD for peak knee extensor (N m kg 1 body mass) from eight landing trials for male and female participants. enced more ankle dorsiflexion and foot pronation, we would expect that they would have experienced less VGRF or, at least, delayed its peak (via greater ankle ROM). These findings may potentially be explained by preplanned muscular landing strategies as the position of the lower extremity at ground contact combined with the muscle activation at this time can influence the magnitude and timing of the VGRF (34). Pflum et al. (30) determined that the ACL load during the landing is largely explained by the magnitude and direction of the resultant GRF relative to the knee joint center throughout the landing phase. Although in vivo human performance studies utilizing methods as presented in the current paper are considered traditional and acceptable techniques in estimating joint kinetics, it is acknowledged that: 1) a skin-based marker TABLE 5. Means of the internal peak joint s between male and female participants in Newton-meters per kilogram of body mass. Mean SD Mean SD Difference Joint Variable Male Female P Ankle Internal plantarflexor 1.99 (0.27) 1.90 (0.39) Internal inversion (0.004) 0.001(0.005) ( )/eversion ( ) Knee Internal extensor 1.75 (0.37) 1.70 (0.27) Internal varus ( )/ 1.61 (0.72) 0.93 (0.69) *** valgus ( ) Hip Internal extensor 1.03 (0.63) 1.01 (0.47) Internal abduction ( )/adduction ( ) 0.47 (0.31) 0.54 (0.26) *** Significant difference in post hoc testing. system and the motion artifact associated with such a data collection scheme may not definitively reflect underlying bone translations and rotations; 2) these errors are further propagated by the estimation of joint forces and s derived from the inverse dynamic approach; 3) the joint forces and profiles reported herein do not represent actual in vivo ACL loads; 4) this study investigated normal landing techniques within male and female participants, and thus, injurious performances cannot be truly assessed, and it is quite plausible that females and males land differently and this performance difference is not a predisposing factor for ACL injury in either group; and 5) although we have made arguments that support and implicate the coupling of the peak knee extensor and the peak knee varus-valgus knee torque as potential determinants of ACL load, we are unaware of published data that have determined how or even when (during the land) an ACL rupture is occurring. Thus, more sophisticated models (25,30,35) are needed to deterministically define ACL load and the individual mechanical components that may contribute to its loading pattern. The females in this study were recreational athletes. Several authors (2,12) have noted skill level as a confounding factor in predisposing females to ACL injury. In the present study, we have assumed that skill levels, defined in terms of: 1) relevant exposure to landings during recreational sports, and 2) previous training and history of participation in sports where landing is common, are not a cause of the observed differences presented herein; although we cannot completely discount skill level as a factor that caused the apparent performance differences, both groups in this study were randomly selected and screened for recreational sports 1010 Official Journal of the American College of Sports Medicine

9 FIGURE 7 Average time normalized curve and SD for knee abduction (N m kg 1 body mass) from eight landing trials for male and female participants (positive values varus, negative values valgus ). participation that included landing types of motions (e.g., high school, varsity-level volleyball, and basketball participation). Due to the exploratory nature of this study, the authors used a rather novel statistical approach in performing a series of 2 3 (group * joint) mixed factor ANOVA with repeated factors. Another 2 3 (group * direction) mixed factor ANOVA with repeated factors was used to analyze the force platform data. Due to the amount of dependent variables associated with just the kinematics of a single limb, this approach was used in an attempt to reduce some of the redundancy of the statistical analyses. Rather than performing a single statistical test on each joint, the entire extremity was analyzed using a two-way ANOVA with repeated factors. The between factor was gender (male vs female) and the within subjects factor was joint (ankle, knee, or hip). Joint was the within subjects factor joint as well as a repeated factor because it was the data from that specific limb. The joints are not viewed as independent because the lower extremity is really a linked system. One joint could influence the outcome of another. This analysis was performed separately for the each plane at initial contact, during maximum and minimum for the joint kinematics and joint kinetics. The same thought process was used to analyze the multiple directions of the ground reaction force data. An REFERENCES 1. ARENDT, E., J. AGEL, and R. DICK. Anterior cruciate ligament injury patternsamong collegiate men and women. J. Athl.Train. 34: 86 92, ARENDT, E., and R. DICK. Knee injury patterns among men and women in collegiate basketball and soccer: NCAA data and review of literature. Am. J. Sports Med. 23: , important limitation of this statistical approach was the inability to test any interaction involving the movement plane. Insights as to the relationship between selected kinetic variables across movement plane were performed through our selected use of correlations. Conclusions. Within the scope and limitations of the present study, the following conclusions were drawn: 1) the majority of the differences in kinematic and kinetic variables between male and female recreational athletes during landing were observed in the frontal plane not in the sagittal plane; 2) females exhibited greater peak knee valgus positions during the land compared with males; 3) after accounting for individual body mass, females generated smaller internal knee varus at the time of peak valgus knee angulation; and 4) the timing of the peak knee extensor torque and the varus-valgus knee torques suggest temporally coupled forces acting at the knee joint and this finding is consistent across both genders. The authors wish to thank Dr. Kevin Shelburne, Ph.D., and Mr. Michael Decker, M.S., for their intellectual contributions, comments, and assistance in the preparation of the final manuscript. This study was supported, in part, by a medical research grant from The NFL Charities (M.R. Torry, Steadman Hawkins Research Foundation) as well as the Graduate School of the University Wisconsin-La Crosse (H. Van Hoof, T. Kernozek). 3. BENDJABALLAH, M. Z., A. SHIRAZI-ADL, and D. J. ZUKOR. Finite element analysis of human knee joint in varus-valgus. Clin. Biomech. (Bristol, Avon). 12: , BUSH, T. R., and P. E. GUTOWSKI. An approach for hip joint center calculation for use in seated postures. J. Biomech. 36: , GENDER DIFFERENCES IN LANDING MECHANICS Medicine & Science in Sports & Exercise 1011

10 5. CHAPPELL, J. D., B. YU, D. T. KIRKENDALL, and W. E. GARRETT. A comparison of knee kinetics between male and female recreational athletes in stop-jump tasks. Am. J. Sports Med. 30: , COLBY, S., A. FRANCISCO, B. YU, D. KIRKENDALL, M. FINCH, JR., AND W. GARRETT. Electromyographic and kinematic analysis of cutting maneuvers. Implications for anterior cruciate ligament injury. Am. J. Sports Med. 28: , DECKER, M. J., M. R. TORRY,D.J.WYLAND, W.I. STERETT, and J. R. STEADMAN. Gender differences in lower extremity kinematics, kinetics and energy absorption during landing. Clin. Biomech. (Bristol, Avon). 18: , DEMORAT, G., P. WEINHOLD, T. BLACKBURN, S. CHUDIK, and W. E. GARRETT. Aggressive quadriceps loading can induce non-contact anterior cruciate ligament injury. Am. J. Sports Med. 32: , DEMPSTER, W.WADC Technical Report. Space Requirements of the Seated Operator. Dayton, OH: Wright Patterson Air Force Base, 1959, pp FORD, K. R., G. D. MYER, and T. E. HEWETT. Valgus knee motion during landing in high school female and male basketball players. Med. Sci. Sports Exerc. 35: , GRAY, J., J. E. TAUNTON, D. C. MCKENZIE, D. B. CLEMENT, J. P. MCCONKEY, and R. G. DAVIDSON. A survey of injuries to the anterior cruciate ligament of the knee in female basketball players. Int. J. Sports Med. 6: , HARMON, K. G., and R. DICK. The relationship of skill level to anterior cruciate ligament injury. Clin. J. Sport Med. 8: , HEWETT, T. E., T. N. LINDENFELD, J. V. RICCOBENE, and F. R. NOYES. The effect of neuromuscular training on the incidence of knee injury in female athletes. A prospective study. Am. J. Sports Med. 27: , HEWETT, T. E., G. D. MYER, and K. R. FORD. Prevention of anterior cruciate ligament injuries. Curr. Women s Health Rep. 1: , HOLLIS, J. M., S. TAKAI, D. J. ADAMS, S. HORIBE, and S. L. WOO. The effects of knee motion and external loading on the length of the anterior cruciate ligament (ACL): a kinematic study. J. Biomech. Eng. 113: , HUSTON, L. J., E. M. WOJTYS, and J. A. ASHTON-MILLER. Gender differences in knee angle when landing from a jump. In: 67th Annual American Academy of Orthopedic Surgeons. Orlando, FL, 2000, pp HUTCHINSON, M. R., and M. L. IRELAND. Knee injuries in female athletes. Sports Med. 19: , IRELAND, M. L. The female ACL: why is it more prone to injury? Orthop. Clin. North Am. 33: , KADABA, M. P., H. K. RAMAKRISHNAN, and M. E. WOOTTEN. Measurement of lower extremity kinematics during level walking. J. Orthop. Res. 8: , KADABA, M. P., H. K. RAMAKRISHNAN, M. E. WOOTTEN, J. GAINEY, G. GORTON, and V. COCHRAN. Repeatability of kinematic, kinetic, and electromyographic data in normal adult gait. J. Orthop. Res. 7: , KIRKENDALL, D. T., and W. E. GARRETT, JR. The anterior cruciate ligament enigma: injury mechanisms and prevention. Clin. Orthop. 372: 64 68, MADIGAN, M. L., and P. E. PIDCOE. Changes in landing biomechanics during a fatiguing landing activity. J. Electromyogr. Kinesiol. 13: , MALINZAK, R. A., S. COLBY, D. KIRKENDALL, and W. E. GARRETT. A comparison of knee joint motion patterns between men and women in selected athletic tasks. Clin. Biomech. 16: , MARKOLF, K. L., D. M. BURCHFIELD, M. M. SHAPIRO, M. F. SHEP- ARD, G. A. FINERMAN, and J. L. SLAUTERBECK. Combined knee loading states that generate high anterior cruciate ligament forces. J. Orthop. Res. 13: , MCLEAN, S. G., X. HUANG, A. SU, and A. J. VAN DEN BOGERT. Sagittal plane biomechanics cannot injure the ACL during sidestep cutting. Clin. Biomech. (Bristol, Avon). 19: , MCLEAN, S. G., P. T. MYERS, R.J.NEAL, and M. R. WALTERS. A quantitative analysis of knee joint kinematics during the sidestep cutting maneuver. Implications for non-contact anterior cruciate ligament injury. Bull. Hosp. Jt. Dis. 57:30 38, MCLEAN, S. G., R. J. NEAL,P.T.MYERS, and M. R. WALTERS. Knee joint kinematics during the sidestep cutting maneuver: potential for injury in women. Med. Sci. Sports Exerc. 31: , MEISTER, K., M. C. TALLEY, M.B.HORODYSKI, P.A.INDELICATO, J. S. HARTZEL, and J. BATTS. Caudal slope of the tibia and its relationship to noncontact injuries to the ACL. Am. J. Knee Surg. 11: , MYER, G. D., K. R. FORD, and T. E. HEWETT. The effects of gender on quadriceps muscle activation strategies during a maneuver that mimics a high ACL injury risk position. J. Electromyogr. Kinesiol. 15: , PFLUM, M., K. B. SHELBURNE, M. TORRY, M. J. DECKER, and M. G. PANDY. Model prediction of anterior cruciate ligament force during drop-landings. Med. Sci. Sports Exerc. 36: , PIZIALI, R. D. NAGEL, T. KOOGLE, and R. WHALEN. Knee and tibia strength in snow skiing. Munich: TUV Publication Series, 1982, pp ROZZI, S. L., S. M. LEPHART, W.S.GEAR, and F. H. FU. Knee joint laxity and neuromuscular characteristics of male and female soccer and basketball players. Am. J. Sports Med. 27: , SALCI, Y., B. B. KENTEL, C. HEYCAN, S. AKIN, and F. KORKUSUZ. Comparison of landing maneuvers between male and female college volleyball players. Clin. Biomech. (Bristol, Avon). 19: , SELF, B. P., and D. PAINE. Ankle biomechanics during four landing techniques. Med. Sci. Sports Exerc. 33: , SHELBURNE, K. B., M. G. PANDY, F. C. ANDERSON, and M. R. TORRY. Pattern of anterior cruciate ligament force in normal walking. J. Biomech. 37: , TRAINA, S. M., and D. F. BROMBERG. ACL injury patterns in women. Orthopedics 20: ; quiz , WOJTYS, E. M., L. J. HUSTON,H.J.SCHOCK,J.P.BOYLAN, and J. A. ASHTON-MILLER. Gender differences in muscular protection of the knee in torsion in size-matched athletes. J. Bone Joint Surg. Am. 85-A: , YU, B., D. KIRKENDALL, and W. E. GARRETT. Anterior Cruciate Ligament Injuries in Female Athletes: Anatomy, Physiology, and Motor Control. Sports Medicine and Arthroscopy Review 10:58 68, YU, B., D. T. KIRKENDALL, T. N. TAFT, and W. E. GARRETT, JR. Lower extremity motor control-related and other risk factors for noncontact anterior cruciate ligament injuries. Instr. Course Lect. 51: , ZHANG, S. N., B. T. BATES, and J. S. DUFEK. Contributions of lower extremity joints to energy dissipation during landings. Med. Sci. Sports Exerc. 32: , Official Journal of the American College of Sports Medicine