In the lower extremity, it is very important to maintain

PREDICTING ASYMMETRICAL LOWER EXTREMITY STRENGTH DEFICITS IN COLLEGE-AGED MEN AND WOMEN USING COMMON HORIZONTAL AND VERTICAL POWER FIELD TESTS: A POSSIBLE SCREENING MECHANISM DAVID W. KEELEY, HILLARY A. PLUMMER, AND GRETCHEN D. OLIVER Department of Health Science, Kinesiology, Recreation and Dance, University of Arkansas Fayetteville, Arkansas ABSTRACT Keeley, DW, Plummer, HA, and Oliver, GD. Predicting asymmetrical lower extremity strength deficits in college aged men and women using common horizontal and vertical power field tests: A possible screening mechanism. J Strength Cond Res 25(6): 1632 1637, 2011 Strength deficits in quadriceps and hamstrings have been linked to several lower extremity injuries. The most common protocol used in testing for se deficits is isokinetic strength testing, which is both costly and time consuming. Therefore, purpose of this study was to employ common vertical and horizontal power field tests to identify those protocols that best predict lower extremity strength deficits. Data describing 22 healthy collegiate graduate students vertical and horizontal power were collected using standard field tests (i.e., 2 feet vertical jump, single leg vertical jump, 40-, 50-, and 60-yd runs). In addition, data describing each subject s lower extremity strength deficits were collected by using Biodex 840-000 Multi Joint System Isokinetic Dynamometer (Biodex Medical Systems, Shirley, NY, USA) set to report peak torque at 60 s 21 of flexion and extension and 180 s 21 of flexion and extension. Regression analyses indicated that 3 of 4 strength deficit parameters could be predicted from a linear combination of field test results (p, 0.05). Of strength deficits measured, hamstring deficits at flexion velocities of both 60 s 21 and quadriceps strength deficits at 60 s 21 were those that could be predicted using field test results. The results of this study, by increasing diagnostic power of clinician, may make it easier to develop strength training protocols designed specifically to target weak musculature in lower extremity. This targeting of specific musculature, in an effort to return symmetrical strength Address correspondence to Dr. Gretchen D. Oliver, goliver@uark.edu. 25(6)/1632 1637 Ó 2011 National Strength and Conditioning Association to lower extremity, may ultimately decrease likelihood of lower extremity injury in college-aged men and women. KEY WORDS isokinetics, strength, vertical jump, 60-yd run INTRODUCTION In lower extremity, it is very important to maintain appropriate strength ratio between hamstring and quadriceps (H/Q ratio), and maintain symmetrical strength between right and left sides. Numerous reports have suggested that asymmetry in lower extremity strength is common among children (12) and adults (2,18). Asymmetrical strength across lower extremities can be defined as inability to produce a force of contraction that is equal across quadriceps and hamstring of both right and left sides. This has been linked to pathological conditions of muscle groups mselves (5) and various lower extremity joints (7). Isokinetic strength testing is commonly used because it allows for full muscle tension throughout range of motion of a joint at a constant speed of movement (10,14). Isokinetic strength testing is used in clinical setting as a primary assessment tool to determine a patient s strength, ability to progress to advanced activities, and ability to return to maximum level of function (1,16). Researchers have used isokinetic strength testing in healthy athletes to relate strength deficits to ir risk of injury. It has been found in soccer players that preseason isokinetic testing can identify individuals at risk for developing hamstring muscle strains (15). Although asymmetrical strength has been linked to a variety of pathological conditions, relatively little research is currently conducted to identify se deficits in collegeaged students. Reasons behind lack of testing may be related to cost, time, and effort currently required to conduct isokinetic strength testing on a regular basis. By developing new methods of testing for lower extremity strength asymmetry, probability of identifying strength deficits may increase. In addition, by determining that se 1632

www.nsca-jscr.org deficits exist at an earlier age, rehabilitation programs can be implemented, which may decrease likelihood of major or permanent injury. One possible method for decreasing cost, time, and effort associated with identifying strength deficits is to employ inexpensive and rapid field tests in an attempt to predict deficits. It has been shown that field tests are both a reliable and effective means for predicting a person s lower extremity strength and power (9). Although se tests have been shown to be effective in predicting lower extremity strength and power, re is currently no known research that investigates effectiveness of specific vertical and horizontal power tests in predicting strength deficits in college-aged men and women. Therefore, purpose of this study was to identify specific field tests for measuring vertical and horizontal power and determine how effective those tests were at predicting asymmetrical strength deficits in lower extremities of college-aged men and women. It was hyposized that some combination of se inexpensive field tests would be capable of predicting lower extremity strength deficits in college-aged men and women. METHODS Experimental Approach to Problem Data describing each subject s vertical power were collected using standard 2 feet vertical jump field test and standard single leg vertical jump field test. Data describing each subject s horizontal power were collected using standard 40-, 50-, and 60-yd run field test. In addition, data describing each subject s lower extremity strength deficits were collected by using Biodex 840-000 Multi Joint System Isokinetic Dynamometer (Biodex Medical Systems, Shirley, NY, USA) set to report peak torque at 60 s 21 of flexion and extension and 180 s 21 of flexion and extension. All testing sessions were completed before 9:00 AM and conducted on separate days giving subjects adequate recovery time. Unfortunately, because of sample available for testing in this study, no dietary controls were implemented. However, all subjects were deemed medically fit for testing and determined to be injury-free based on a completed medical history form. Additionally, all subjects included in study were deemed to be moderately physically fit based on results of completed Wingate and _ VO 2 max testing. After completion of all data collection sessions, data were analyzed using a single group stepwise regression model designed to identify those parameters that were best capable of predicting strength deficits. Before conducting regression analysis, all data were analyzed using a series of descriptive statistics to identify outliers and determine nature of distribution before testing for presence of relationships. Once all data were deemed to be normally distributed, a backward stepwise regression analysis was conducted to identify predictors of lower extremity strength asymmetry. In current design, vertical and horizontal power field tests were independent variables, whereas quadriceps and hamstring strength deficits were dependent variables. These variable assignments are consistent with development of a model designed to identify parameters capable of predicting specific outcomes. Data analysis for this study was conducted using statistical analysis package SPSS 11.5 for Windows (SPSS, Chicago, IL, USA). Subjects Twenty-two healthy collegiate graduate students (age: 23.36 6 2.36 years, height: 172.33 6 10.48 cm, and mass: 79.76 6 16.43 kg), regardless of gender, consented to participate in this study. Data collection sessions were conducted on campus at University of Arkansas. All testing protocols were approved by Institutional Review Board. These approved procedures were explained to all subjects before testing. Subjects also provided consent based on ir understanding of se procedures before participating in any testing sessions. Bilateral Isokinetic Testing of Knee Before collection of vertical and horizontal power data, Biodex 840-000 Multi Joint System Isokinetic Dynamometer (Biodex Medical Systems) was set up to collect data about knee joint at resistive speeds equal to 60/180 s 21. After subjects had completed a warm-up program of ir choice, y were positioned for test trials in following manner: Per instructor recommendations, both dynamometer and dynamometer seat were positioned at 45 and appropriate leg attachment was affixed to dynamometer. In addition, seat was also situated in such a manner that a subject s knee was positioned adjacent to dynamometer fulcrum and ir thigh did not extend off seat and seatback tilt ranged between 70 and 85. Although position of a subject s knee was such that it was close to dynamometer, full range of motion was still achievable. To secure a subject into seat, ipsilateral thigh, pelvis, and torso were secured using various straps. The contralateral thigh was not secured during testing (3). After subject had been secured, and was ready to complete test trials, shaft of dynamometer attachment was attached to shank of leg for which data were to be collected using a strap/pad fastened tightly at ankle just superior to malleoli. After a subject had been properly positioned and was ready to proceed with test trials, it was necessary to select 60/180 s 21 protocol and calibrate machine by establishing subject specific range of motion (ROM) limits and weigh limb to be tested. To establish AWAY ROM limit, subject was asked to fully extend his or her knee before researcher placed machine in hold mode to set limit. Similarly, to establish TOWARDS ROM limit, dynamometer was taken out of hold mode and subject was asked to fully flex knee. Once knee was positioned at angle of greatest flexion, dynamometer was again placed in hold mode to set limit. Once both VOLUME 25 NUMBER 6 JUNE 2011 1633

Predictors of Lower Extremity Strength Deficits TABLE 1. Mean and SD for vertical jumping and timed run field tests.* TABLE 3. Model summary for quadriceps strength deficit at 60 s 21.* Variable Results (mean 6 SD) Model R R square Adjusted R square SEE Vertical jump tests (cm) Two feet (n = 22) 51.44 6 13.30 Right foot (n = 22) 44.33 6 11.58 Left foot (n = 22) 44.51 6 12.84 Timed run tests (s) 40 yd (n = 21) 5.82 6 0.71 50 yd (n = 21) 6.99 6 0.82 60 yd (n = 21) 8.43 6 1.10 *Vertical jumping test results are broken down by jump type and are displayed in centimeters. Timed run tests results are broken down by distance and are displayed in seconds. TOWARD and AWAY limits were established, dynamometer was calibrated by having subject relax and placing dynamometer in hold mode when shank was positioned orthogonally to thigh. Finally, to complete dynamometer setup, subject was asked to fully extend his or her leg; dynamometer was placed in hold mode and limb was weighed (3,4). Upon completing dynamometer setup, computer monitor was turned away from subject s visual field, and test trials were begun. Test trials consisted of 1 set of 5 repetitions at a resistive speed of 60 s 21 and 1 set of 5 repetitions at a resistive speed of 180 s 21. Before each test set, subjects were allowed to perform practice repetitions to acclimate mselves to resistance level. To begin test trials, subjects were required to hold leg at TOWARD limit for a period of time. During test trials, subjects were TABLE 2. Mean and SD results for isokinetic strength testing.* 1 0.598* 0.358 0.283 6.62 *Predictors: (constant), 40-yd dash, 60-yd dash. verbally encouraged to perform at ir maximum for all repetitions and to flex and extend leg through full ROM. Between each set, a rest and recovery period of 15 seconds was provided. Upon completing all test sets for 1 leg, testing protocol was repeated for opposite leg. Neir ROM limits nor leg weights were carried over from 1 leg to anor. All testing protocols were performed independently for each leg. Vertical Power Testing To collect data describing vertical power, a Vertec (Questtek Corp. Northridge, CA, USA) vertical reach and jumptesting apparatus was used. To determine vertical reach height (standing height), subjects were asked to stand directly beneath apparatus with ir dominant arm held above ir shoulder in a position of maximal flexion and abduction, with ir hand in a neutral position, and with ir fingers extended fully. The lowest movable Vertec marker was n positioned so that it contacted longest extended finger without resulting in displacement of any portion of subject s body. Once testing apparatus was set up, a series of 9 test trials (3 standard vertical jumps, 3 preferred leg vertical jumps, and 3 nonpreferred leg vertical jumps) were conducted. For 3 standard vertical jump trials, subjects were allowed to perform a short countermovement before jumping. The countermovement was such that subjects were allowed to squat to a depth of ir choice. During jump portion of trial, subjects were asked to reach to a position of maximal flexion and abduction, with ir hand in a neutral position, and with Variable Results in % (mean 6 SD) Extension deficits (%ftlb) 60 s 21 (n = 21) 23.60 6 7.65 180 s 21 (n = 21) 0.99 6 11.22 Flexion deficits (%ftlb) 60 s 21 (n = 21) 26.69 6 8.88 180 s 21 (n = 21) 0.95 6 15.32 *Results are displayed as percentage of net strength deficit (positive values indicate stronger nonpreferred leg/ negative values indicate stronger preferred leg) in flexion and extension at both 60 and 180 s 21. TABLE 4. Overall model testing results for quadriceps strength deficit at 60 s 21.* Model SS df MS F Sig. 1. Regression 415.78 2 207.89 4.744 0.023* Residual 745.03 17 43.83 Total 1,160.81 19 *Predictors: (constant), 40-yd dash, 60-yd dash. SS = sum of squares; MS = mean of squares. 1634

www.nsca-jscr.org TABLE 5. Results of regression coefficient testing for quadriceps strength deficit at 60 s 21. Model Nonstandardized coefficients ir fingers extended fully to displace highest possible Vertec marker. For each trial, height of displaced marker was recorded, and trial resulting in highest vertical jump was prepared for furr analysis. In same fashion, 3 single leg vertical jump tests were also conducted for each of right and left foot. Horizontal Power Testing To determine a subject s horizontal power, timed run tests were used. Before conduction of each horizontal power test trial, subjects were instructed to move to a position behind start line and were allowed to begin with a staggered, upright start, and with ir preferred leg forward. All subjects were allowed to initiate ir own test trials as y deemed mselves ready. As subjects ran, times were collected at 40, 50, and 60 yd and at conclusion of 60 yd, subjects had been instructed to run through stop line in an attempt to improve ir time. All subjects completed 2 60-yd test trials. Each subject s times were recorded for all trials, and trial resulting in fastest time was prepared for furr analysis. Statistical Analyses Before any predictive statistical analysis procedures, data in current study were analyzed from a descriptive standpoint. A set of descriptive statistics were conducted to identify possible presence of outliers (z-scores) and to determine nature of distributions (i.e., skewness and kurtosis) before testing to identify predictive models. Once descriptive analyses were concluded and distributions of data were determined to be approximately normally distributed, multiple regression techniques were employed to determine predictive nature of common field test used in study. In regression analyses, independent variables were field power tests and isokinetic strength deficits were dependent variables. These assignments allowed for development of models that would use field test results to predict lower extremity strength deficits in college-aged subjects. Standardized coefficients B SE Beta t Sig 1. Constant 23.796 12.26 1.423 20.31 0.761 40 15.31 5.32 21.522 2.88 0.01 60 210.55 3.43 23.08 0.007 RESULTS Vertical and Horizontal Power Field Tests The results of testing describing vertical (vertical jump tests) and horizontal (timed run tests) power are displayed in Table 1. It is important to note that re was 1 case of experimental mortality during timed run tests because of a previous injury, and thus, number of participants was different throughout course of conducting various vertical and horizontal power tests. Isokinetic Strength Results of isokinetic strength testing are displayed in Table 2. These results indicate that on average, college-aged men and women experience some measure of muscular strength deficit for both quadriceps and hamstrings. This study defined a strength deficit as percentage of peak torque difference between preferred and nonpreferred legs. Mamatically, this was determined by dividing lowest magnitude of peak torque by highest magnitude of peak torque across isokinetic testing trials. When preferred leg generated a higher value of peak torque, strength deficit was negative. When nonpreferred leg generated a higher value of peak torque, strength deficit was positive. One finding of interest is that sample of college-aged students in this study generated greater peak flexion and extension torques for preferred leg at 60 s 21. In contrast, at TABLE 6. Model summary for hamstring strength deficit at 60 s 21.* Model R R square Adjusted R square SEE 1 0.621* 0.386 0.314 9.51 *Predictors: (constant), 2 foot vertical, 60-yd dash. TABLE 7. Overall model testing results for hamstring strength deficit at 60 s 21. Model SS df MS F Sig. 2. Regression 966.36 2 483.18 5.340 0.016* Residual 1,538.12 17 90.48 Total 2,504.47 19 *Predictors: (constant), 2 foot vertical, 60-yd dash. VOLUME 25 NUMBER 6 JUNE 2011 1635

Predictors of Lower Extremity Strength Deficits TABLE 8. Results of regression coefficient testing for hamstring strength deficit at 60 s 21. Model 180 s 21, sample generated greater peak flexion and extension torques for nonpreferred leg. Regression Results of regression analysis indicate that 3 of 4 strength deficit parameters could be predicted from a linear combination of field test results. Of 2 extension deficit parameters, only regression model developed for predicting quadriceps strength deficits at 60 s 21 retained any independent variables. The overall model summary for predicting quadriceps strength deficits is shown in Table 3. The results of overall model significance testing is shown in Table 4, and results of regression coefficient testing are shown in Table 5. The final model predicting quadriceps strength deficits at 60 s 21 is defined using equation 1 and explained 36% of variance in quadriceps strength deficits (r 2 = 0.359): Y ¼ð15:31Þð40 ydþþð 10:55Þð60 ydþ 3:80: ð1þ Just as with quadriceps strength deficits, only 1 of 2 regression models was significantly capable of predicting strength deficits for hamstrings. The model capable of predicting strength deficits in hamstrings were identified at both 60 s 21.For60 s 21, overall model summary for predicting hamstring strength deficits is shown in Table 6. The results of overall model significance testing is shown in Table 7 and results of regression coefficient testing are shown in Table 8. The model predicting strength deficits at 60 s 21 is defined in equation 2 and explained 39% (r 2 =0.386) of variability in hamstring strength deficits at 60 s 21 : Y ¼ 0:629 ð2 foot vertical jumpþþð 10:30Þð60 ydþ þ 119:11: ð2þ Additionally, although hamstring strength deficits observed at a velocity of 180 s 21 could not be significantly predicted, best model developed (p =0.06,r 2 = 0.27) did retain same combination of variables as model at 60 s 21. DISCUSSION Nonstandardized coefficients Asymmetrical strength in lower extremity, which is defined as inability to produce a force of contraction that is Standardized coefficients B SE Beta t Sig 1 Constant 119.11 39.10 21.011 3.05 0.007 Forty 210.30 3.19 20.704 23.23 0.005 Sixty 20.63 0.28 22.24 0.038 1636 equal across quadriceps and hamstring of both preferred and nonpreferred sides, has been linked to a number of lower extremity problems (5,7). Although se strength deficits have been linked to a variety of pathological conditions, relatively little research is currently conducted to identify se deficits early in a person s life. Reasons for this lack of testing may be related to cost, time, and effort currently required to conduct isokinetic strength testing on a regular basis. This study works to develop new, less time-consuming methods for increasing ability of a clinician to identify se deficits. Research has recently been completed by analyzing different methods of measuring strength imbalances of lower extremity. It was found that re were moderate correlations with squat and vertical jump force measures of imbalance, suggesting that this simple field test may have potential for assessment of leg strength imbalance (13). The results of this study indicate that variability in lower extremity strength and as such possible strength deficits in college-aged men and women may be identified using a variety of common field tests. These field tests, which include both timed run tests and 2 foot vertical jumping test, can be used in regression equations to calculate possible strength deficits. These models may have potential to increase a clinician s ability to screen for possible strength asymmetries in both lower extremity flexors and extensors. By increasing diagnostic power of clinician, se models, when used as a screening tool, may make it easier to identify those students in need of more indepth strength testing. PRACTICAL APPLICATIONS The results of this study indicate that potential exists for athletic trainer to quickly screen for lower extremity strength deficits in athletic populations. In addition, se results may also potentially provide clinicians with an opportunity to screen for similar deficits in athletic population. Inadequate rehabilitation and premature return to play after injury have been suggested risk factors for recurrence of injury in previous studies (8). The potential to screen for lower extremity strength deficits may also provide athletic trainers and clinicians alike with a new tool in ir efforts in preventing injury and determining an athlete s ability to safely return to play after injury. As with or factors that contribute to injury, early identification may be key for preventing lower extremity problems associated with asymmetrical strength in quadriceps and hamstrings. It has been proposed by clinicians that strength deficits in weakest leg should be

www.nsca-jscr.org restored to within 10% of that of unaffected leg (11,17). This study, by employing field tests that are both easy to conduct and commonly used, may help clinicians screen for lower extremity strength deficits, and develop training routines designed to reduce observed deficits. For instance, it has been found that professional soccer players with untreated muscle imbalances were 4 5 times more likely to sustain a hamstring injury than ir counterparts with symmetrical muscle strength (6). By incorporating quick, easy field tests as opposed to time consuming isokinetic testing protocols, strength deficits in populations such as se may be quickly screened for, and a determination as to need for furr testing identified significantly earlier. Additionally, using se predictive models, appropriate strength training routines may be created in an attempt to reduce chances of developing common problems associated with asymmetrical strength in lower extremity. ACKNOWLEDGMENTS No authors received financial support for this study. REFERENCES 1. Andrews, JR and Harrelson, GL. Physical Rehabilitation of Injured Athlete. Philadelphia, PA: W.B. Saunders Company, 1991. 2. Askling, C, Karlsson, J, and Thorstensson, A. Hamstring injury occurrence in elite soccer players after preseason strength training with eccentric overload. Scand J Med Sci Sports 13: 244 250, 2003. 3. Biodex Multi-Joint System Setup/Operation Manual. Shirley, NY: Biodex Medical Systems, 2007. pp. 32 35. 4. Brown, E, Whitehurst, M, Bryant, JS, and Buchalter, DN. Reliability of Biodex system isokinetic dynamometer concentric mode. Isokinet Exerc Sci 3: 160 163, 1993. 5. Burkett, LN. Causative factors in hamstring strains. Med Sci Sport Exerc 2: 39 42, 1970. 6. Croisier, JL, Ganteaume, S, Binet, J, Gentry, M, and Ferret, JM. Strength imbalances and prevention of hamstring injury in professional soccer players. Am J Sports Med 36: 1469 1475, 2008. 7. Gribble, PA and Robinson, RH. An examination of ankle, knee, and hip torque production in individuals with chronic ankle instability. J Strength Cond Res 23: 395 400, 2009. 8. Haggulund, M, Walden, M, and Ekstrand, J. Injury incidence and distribution in elite football-a prospective study of Danish and Swedish top divisions. Scand J Med Sci Sports 15: 21 28, 2005. 9. Hamilton, RT, Shultz, SJ, Schmidt, RJ, and Perrin, DH. Triple-hop distance as a valid predictor of lower limb strength and power. J Athl Train 43: 144 151, 2008. 10. Hislop, JH and Perrine, JJ. Isokinetic concept of exercise. Phys Ther 47: 114 117, 1967. 11. Kannus, P. Isokinetic evaluation of muscular performance. Int J Sports Med 15: S11 S18, 1994. 12. Molnar, GE and Alexander, J. Objective quantitative muscular testing in children: A pilot study. Arch Phys Med Rehab 57: 224 228, 1974. 13. Newton, RU, Gerber, A, Nimphius, S, Shim, JK, and Doan, BK. Determination of functional strength imbalance of lower extremities. J Strength Cond Res 20: 971 977, 2006. 14. Nunn, KD and Mayhew, JL. Comparison of three methods of assessing strength imbalance at knee. J Orthop Sports Phys Ther 10: 134 137, 1988. 15. Orchard, J, Marsden, J, Lord, S, and Garlick, D. Preseason hamstring muscle weakness associated with hamstring muscle injury in Australian footballers. Am J Sports Med 25: 81 85, 1997. 16. Shelbourne, KD and Nitz, P. Accelerated rehabilitation after anterior cruciate ligament reconstruction. Am J Sports Med 18: 292 299, 1990. 17. Sherry, MA and Best,. A comparison of 2 rehabilitation programs in treatment of acute hamstring strains. J Orthop Sports Phys Ther 34: 119 125, 2004. 18. Wyatt, MP and Edwards, AM. Comparison of quadriceps and hamstring torque values during isokinetic exercise. J Orthop Sport Phys 3: 48 56, 1981. VOLUME 25 NUMBER 6 JUNE 2011 1637