The higher incidence of female anterior cruciate ligament (ACL) injuries compared with their male counterparts is well documented (24). Among the many risk factors investigated that are associated with ACL injury, lower extremity kinematics has been suggested to be a significant contributor (29). Although not limited to these kinematic risk factors, women tend to land with less knee and hip flexion and higher knee valgus angles (13,16,26) and squat with more hip adduction (29). These positions increase the loads and stress placed on the ACL with reduced muscular support to absorb the landing force (5), which may occur as a compensation for low levels of strength. This suggestion is supported by reduced hip and knee flexion and high incidence of ACL injury during unilateral landings (23,27) that occur with reduced capacity to produce force and control the impact in comparison with landing on both legs.
A lack of strength has been reported to be a risk factor for ACL injury, which is supported by a lower strength to mass ratio found in Division I female athletes compared with recreational male athletes matched for activity level (16). Strength training is commonly prescribed to prevent ACL injury in combination with other types of exercise (9). Plyometrics, agility, balance, technique, and flexibility have also been incorporated in ACL prevention programs (9,11,22). Although mixed results have been reported, improvements in these components of fitness have shown to reduce the incidence of ACL injuries in several investigations (9,10,22). Research demonstrating positive results have combined several of the components as a part of the training program (9,10,17), typically analyzed before and after 8 weeks of training (22). Although analyzing the training effect of several types of training within a particular group can improve our understanding in the prevention of knee injuries, the effect each component has on knee injury prevention is currently unclear. To better understand the role of improved strength on lower extremity kinematics, the investigation of strength training alone is warranted.
Inconsistent results have been found in the few investigations that have isolated the use of strength training to analyze changes in lower extremity kinematics. Variation in the mode of resistance and the task analyzed might have contributed to the different findings. Herman et al. (8) found no change in hip and knee kinematics during a stop-jump task after non–weight-bearing resistance-band training in female recreational athletes. The authors noted that strength gained in a non–weight-bearing exercise may not transfer to improved performance in a weight-bearing task. In contrast, Lephart et al. (15) found increased hip and knee flexion after a vertical countermovement jump and landing task in high school female athletes after an 8-week training period, but no change occurred in knee valgus angle. The type of resistance training completed was not clearly described in this study. The increased joint flexion was suggested to reduce the risk of ACL injury by creating a soft landing (reduced ground reaction forces with longer absorption time) with increased muscle use and less stress on the ACL. Because of the necessity to control the body and forces in space during maneuvers that occur during sport activity, free weight exercise is arguably the most common and most important type of resistance training used by athletes to increase strength and power and prevent injury. Thus, the use of weight-bearing free weight exercises on lower extremity kinematics in athletic populations requires further investigation.
Previous studies have found significant correlations between joint strength and lower extremity kinematics during sport maneuvers (12,13). Gluteus medius activity is suggested to prevent excessive hip adduction, whereas hamstring strength controls anterior tibial shear and frontal plane knee stability (18,29). These studies indicate that lower extremity control and stability during athletic maneuvers can be enhanced with strength improvement; however, further training investigations are needed to determine if kinematic changes related to ACL injury prevention occur in female athletes after training. Therefore, the purpose of this study was to determine the effect that weight-bearing free weight training alone has on hip and knee flexion and knee valgus angles. With a high incidence of ACL injury occurring while landing in female athletes, the second purpose was to compare unilateral and bilateral landing kinematics. With stronger correlations between landing kinematics and hip strength found in female athletes (13), we hypothesized that the training program would increase hip and knee flexion and decrease knee valgus. With less musculature support and consequently lower strength levels applied during unilateral vs. bilateral drop jumps, we hypothesized that reduced joint motion would occur during unilateral drop jumps.
Experimental Approach to the Problem
Research supports that strength training in combination with other modes such as plyometrics, agilities, and balance training (9) is effective for changing lower extremity landing kinematics and reducing the incidence of ACL injuries (22). However, the effect each component has alone on kinematic change is not clearly understood. To better understand the effect strength training has on lower extremity kinematics during drop jumps, strength training alone must be investigated. This information will enhance the ability of strength and conditioning specialist to determine to degree of emphasis placed on strength training to alter landing kinematics in female athletes. Recent studies investigating the effect of strength training alone on these kinematic measures have revealed mixed results (8,15). These investigations have not included the analysis of functional strength training typically used by athletes. Therefore, strength training with bilateral and unilateral free weight resistance in a weight-bearing stance was implemented because these are the most common resistance exercises used by athletes. These exercises were also included because of the limited research investigating the effect of resistance exercises that are mechanically similar to drop jumps. To further determine the effect of strength on landing kinematics, unilateral and bilateral drop jump kinematics were also compared. Less strength from the use of less muscle support is capable during unilateral drop jumps, which may produce different kinematics in comparison with bilateral drop jumps.
Because of the high incidence of ACL injury in female athletes, female recreational athletes were divided into resistance training group and control group, who did not perform lower-body resistance training. The subjects completed pre- and posttest to measure hip and knee kinematics during unilateral and bilateral drop jumps. An 8-week training program was implemented, to follow previous research that has analyzed the effect of strength improvement on kinematic changes during landing (15). Strength improvement have been shown to occur within an 8-week training period (15) with neural adaptations being the likely mechanism to improve strength in the initial weeks of training, whereas hypertrophy provides contribution near the end of the 8 weeks of training. A linear periodized program was implemented with the intention to improve lower-body weight-bearing strength of the participants during the 8 weeks of training and determine if improved strength alters hip and knee flexion and knee valgus angles during a drop jump. These joint angles analyzed during drop jumps are associated with the risk of ACL injury (29).
Twenty-seven healthy female subjects with previous high school athletic experience (height, 163.07 ± 8.74 cm; mass, 63.64 ± 8.25 kg; age, 21.04 ± 1.83 years) were included in this study. The subjects were screened for injuries and excluded for any physical condition that would have affected the training and jump performance. The subjects were recreationally active in a variety of sports (soccer, basketball, volleyball, and softball) with a minimum participation of 1 time per week before the beginning of the study. All subjects had a minimum of 1 year of previous resistance training experience and previous participation on a high school athletic team. All subjects were advised regarding the testing methods and procedures of the study. Each subject read and signed an informed consent document after a review and approval of the investigation by the internal review board for use of human subjects in research at the university from which the study originated.
Jump and Squat Assessment
One practice session took place to familiarize the subjects with the procedures and to practice the jump techniques. The subjects were instructed to refrain from exercise 48 hours before the test sessions and to eat breakfast approximately 2 hours before testing. All drop jumps occurred during a similar time of the day. For the drop jumps, the subjects stepped off of a box with hands on the hips (60 cm for the bilateral jumps and 30 cm for the unilateral jumps) and dropped to the floor without stepping down or jumping up before the drop. Upon landing, the subjects were instructed to jump for maximum height. All jumps were completed in 1 session in random order after a 5-minute jog and light lower-body stretching. The dominant leg (swing leg during a kick) was analyzed during the unilateral and bilateral jumps. The subjects completed 3 trials for each type of jump with 30 seconds of rest between the trials and a 2-minute rest period between each test. To eliminate potential learning effect across trials, 3 practice trials took place. Maximum and mean knee valgus, knee flexion, and hip flexion angles from the 3 trials were analyzed.
Two practice sessions were devoted to technique on all exercises and to estimate loads that could be performed for 15 repetitions on each exercise (lunge, Romanian deadlift, step-up, and unilateral squat). A 1 repetition maximum (1RM) bilateral squat assessment took place after a minimum of 48 hours of recovery. The subjects were instructed to perform a 5-minute jog as a warm-up exercise and light stretches for the lower extremity before testing. The subjects performed each lift using an audio feedback monitor (Bigger Faster Stronger, Salt Lake City, UT, USA) across the middle of the thigh that was activated when the thigh reached a parallel position. Before these tests, a manual goniometer was used to determine that the audio monitor was activated when the femur was at the parallel position. The subjects were instructed to maintain a natural curve in the low back with the chest up to eliminate excessive trunk flexion during the squat. These positions were subjectively analyzed and included in the criteria to determine a successful trial during the strength assessments. The subjects were allowed to use their selected stance width and bar placement. Two warm-up sets were completed using light loads for 3–5 repetitions. The following sets were considered 1RM trials. An initial trial was approximately 85% of the subject's estimated 1RM. With each successful trial, 10–20% was added to the bar. All subjects completed the 1RM test within 4 trials. These tests were repeated as a posttest.
Kinematic data were collected using the MotionMonitor motion capture system (Innovative Sports Training, Inc., Chicago, IL, USA). Electromagnetic sensors were placed on the sacrum, thigh, and shank with double-sided tape and covered with athletic prewrap and tape on the right leg. The world and segment axis setup used a right hand coordinate system with the positive x-axis leftward, positive y-axis forward, and negative z upward. Knee and ankle joint centers were determined by the digitized center between the medial and lateral femoral condyles and the medial and lateral malleoli, respectively. Hip joint center was determined by digitizing the right and left anterior superior iliac spine using the method by Bell et al. (2). Euler angle sequence that rotated in the order of flexion-extension (x axis), valgus-varus (y axis), and internal-external rotation (z axis) was used to calculate joint angles. Data were sampled at 100-Hz filtering at 10 Hz using a low-pass fourth-order Butterworth filter (3).
After the pretests, the subjects were randomly divided into a control group (no resistance training) and an experimental group (resistance training). Training took place 2 days per week for 8 weeks. All exercises were completed using weight-bearing free weight resistance. Before each exercise session, 5-minute general warm-up was used to increase the heart rate and blood flow to the tissues followed by a dynamic warm-up for the lower extremity. Each resistance exercise was preceded by a warm-up set using light resistance for 5–10 repetitions. A linear periodized program was implemented by increasing the percentage 5% each week while decreasing the volume. Progression was also implemented by adding exercises during the training period with the bilateral squat and Romanian deadlift included in the initial weeks and for the entire 8 weeks. Unilateral exercises were added in the middle of the training period. Two to 4 sets were completed for all exercises with 2–3 minutes of rest between each set and exercises. The bilateral squat was completed before the unilateral exercises, and the Romanian deadlift was completed last. A 5-minute cooldown of stretching took place at the end of each session that focused on the hip and knee. The bilateral squat was performed in a squat rack with spotters who were also used for the lunge and step-up. The lunge and step-ups were completed with a bar loaded with free weights. A 12-inch step was used for the step-up, whereas a dumbbell was used to add weight for the unilateral squat. For the 2 sessions performed each week, the same exercise prescription was completed. Any training session missed was completed within 2 days after the normal training session. All training sessions took place at the same time of the day and were monitored by the researchers. Table 1 provides the specific sets, repetitions, percentage, and the week each exercise was added to the training program. For the bilateral squat, the intensity ranged from 50% to 85% of the subject's 1RM from the first to the final week of training while using 12 repetitions during the first week and 4 repetitions at the highest loads in the final week. For all other exercises (lunge, step-up, unilateral squat, and Romanian deadlift), the repetitions ranged from 4 to 12, which were completed using a repetition maximum (6–15RM) that required maximum effort to complete all repetitions.
The dependent variables in this study were measures of knee valgus, knee flexion, and hip flexion during a drop jump task. A repeated measures analysis of variance was used to determine differences across training groups (control vs. resistance trained), type of jump (unilateral vs. bilateral), and trials (pretest vs. posttest). The training group variable was the only between-subjects factor. The type of jump and the trial variables were both within-subjects factors. Interactions among all factors were tested. Mean and maximum values were analyzed separately for each dependent variable. The level of significance was set at p ≤ 0.05. The test-retest intraclass correlation coefficients ranged in the order of 0.83 ≥ r ≤ 0.99 across the dependent variables.
No significant main effects for training groups (control vs. resistance trained) or trials (pretest vs. posttest) were observed. No significant 2-way interactions between training group by trial or jumping condition (unilateral vs. bilateral) by trial were observed for any variable. However, a 3-way interaction (group × jumping condition × trial) was observed (F(1,24) = 5.8; p = 0.025) for knee flexion only. For the control group, there was no significant difference between the pre- and posttest values for knee flexion for the unilateral jump, but there was a significant decrease from pretest (82.4 ± 3.9) to posttest (69.6 ± 5.2) values for the bilateral jump. The same interaction was not observed for the resistance-trained group. For this group, even though there was no significant difference between the pre- and posttest values for knee flexion when jumping on 1 leg, there was a significant increase from pretest (77.2 ± 4.1) to posttest (83.2 ± 3.7) values when jumping on 2 legs. Means and SEs are reported in Table 2.
For both the mean and maximum values of each dependent variable, the bilateral measures were significantly (p < 0.05) greater than the unilateral measures during the drop jump task. Significant differences were observed between unilateral (11.2 ± 1.0) vs. bilateral (12.5 ± 1.1) mean knee valgus (p = 0.03), unilateral (12.5 ± 1.0) vs. bilateral (14.2 ± 1.2) maximum knee valgus (p = 0.01), unilateral (52.3 ± 1.9) vs. bilateral (77.9 ± 2.2) mean knee flexion (p < 0.0001), unilateral (58.5 ± 2.1) vs. bilateral (84.8 ± 2.3) maximum knee flexion (p < 0.0001), unilateral (34.2 ± 1.5) vs. bilateral (51.4 ± 1.6) mean hip flexion (p < 0.0001), and unilateral (37.9 ± 1.6) vs. bilateral (56.4 ± 1.6) maximum hip flexion (p < 0.0001).
The increased knee flexion found in this study would indicate a favorable reduction in the risk of an ACL injury during a drop jump task. This would indicate that less stress was placed on the ACL because of a likely softer landing with more reliance on muscular effort to control the joint through the descent. Women tend to land stiff with less hip and knee flexion angles and higher normalized ground reaction forces while landing (10,26) with a shorter deceleration phase in comparison with men (10,16). The data suggest that this landing strategy is altered at the knee after strength improvement, which was likely because of neural adaptations in the initial weeks of training, while hypertrophy providing equal contribution in the final weeks. No change in knee flexion occurred during the unilateral drop jump. The unilateral squat was included in the training program after week 4. It is possible that a longer training period using unilateral exercises is needed to increase unilateral strength to a level that will alter knee kinematics during a unilateral drop jump.
No change occurred for hip flexion for the unilateral and bilateral drop jump. Athletes appear to have neuromuscular programs for kinematic control that may be unaffected by strength training alone. The addition of other types of training such as proper landing technique instruction (21) with strength training may be necessary to alter the hip kinematics. With the tendency to land with less hip flexion and a more upright posture (5), women may rely more on the improved strength gained in the quadriceps to produce the softer landing, indicated by the increase in knee flexion. However, with no change in hip flexion, loads placed on the ACL may not be reduced even with the increase in knee flexion, as the upright posture likely remained unchanged. An upright landing posture produces higher ground reaction forces and quadriceps activity compared with an increase in trunk and hip flexion (4), which is reported to be a risk factor for ACL strain and injury (3). Previous research has shown that women produce higher quadriceps activity than their male counterparts during landing (6) and primarily use the ankle and knee to absorb landing forces (5). The data in this study support that this landing strategy remains unchanged with increased strength and indicate that as squat strength increased, enhanced reliance on the knee musculature occurred to control the landing.
With previous studies indicating that the hip musculature is not the primary joint used to absorb landing forces in female athletes (5), the Romanian deadlift was included to target hamstring activity and improve sagittal plane strength at the hip. Strain on the ACL may have been reduced with enhanced hamstring activity to counter any potential increase in quadriceps activity (1) for anterior tibial translation control and reduction in knee adduction-abduction moments (11,18). Further studies are needed to analyze hip musculature activity and ACL strain during landing after training.
Although unknown in this study, the production of forces at different joint velocities between strength training and a landing task may have contributed to the inability of the improved strength to alter hip kinematics. Resistance training at higher velocities may be needed to alter kinematics during the drop jump. The neuromuscular adaptations occurring from high-velocity training, such as plyometrics, have been shown to differ in comparison with strength training (15). Plyometric training has reduced ground reaction landing force (11) and enhanced neuromuscular activation of the lower body before landing, whereas strength training did not demonstrate this adaptation (15). Early activation of the musculature before landing would indicate an improved joint control at the initial contact that may continue through the entire eccentric motion until peak joint angles occur and through the concentric phase of the jump. Future research is needed to analyze the effect of combining plyometric and strength training and each in isolation on drop jump kinematics.
Valgus angle did not change after strength training, which is in agreement with previous research (15). Lephart et al. (15) found no change for a bilateral vertical jump and landing task in high school female athletes after 8 weeks of resistance training. The program consisted of completing 20–30 repetitions for each resistance exercise with an increase in repetitions for progression, but a description was not provided for the type of exercise or intensity. Our results are also in agreement with Herman et al. (8) who found no change in valgus knee angles after non–weight-bearing single-joint training using band resistance. Although multijoint weight-bearing training with free weights is considered more sport specific than band training, it appears that both types of training do not alter knee valgus angles during landing tasks. In contrast to our findings, Mascal et al. (19) found reduced knee valgus motion during a step-down task in 2 case studies who were untrained and had patellofemoral pain. The strength of these subjects increased 50–110% in selected muscle groups after resistance training, which indicated significant muscle weakness before training. These previous data indicate that knee valgus motion may be altered in highly untrained subjects who make large improvements in strength. The resistance training experience of the subjects was not provided by Lephart et al. (15), while no resistance training took place for 12 weeks before the study by Herman et al. (8). The subjects in our study had a mean of 4.3 years of training experience with the majority of the training occurring during high school. Although 1RM squat increased 19% (55.97–69.18 kg), further gains may be required to significantly alter frontal plane kinematics during a drop jump task in athletes with previous resistance training experience. With only 2 sessions per week for 8 weeks, more time training with heavy loads, most effectively used to maximize strength gains, may be necessary to affect landing kinematics.
Long-term resistance training with a focus on increasing strength in the frontal plane may be necessary to alter frontal plane kinematics. Valgus loading of the knee is a strong predictor of ACL injury (10). Previous research has shown that exercises with a reduced medial-lateral base of support compared with the traditional bilateral squat produce higher activation in the musculature controlling frontal plane motion (14,20). In addition, an association between hip abduction strength and knee abduction angle has been demonstrated during running (7) and landing (12,13) in women. Hip abductor strength has also demonstrated a higher association to knee landing kinematics in healthy adult women compared with men (13). In addition, an increase in knee valgus angle has been shown to occur after hip abductor fatigue in a group of recreationally active men and women (28). The bilateral squat, which trains the musculature primarily in the sagittal plane, was included for the entire 8 weeks, whereas the unilateral squat, step-up, and lunge were introduced in the middle of the 8-week program. These frontal plane exercises may need to be included for a longer training period.
In comparison with bilateral drop jumps, less hip and knee flexion and valgus angles were produced during the unilateral drop jump, which indicate that these drop jumps have a higher risk of ACL injury. Weinhandl et al. (27) found similar results in male and female athletes who landed from a drop height equal to their jump height and produced reduced knee flexion and knee abduction during unilateral landings. These subjects were required to perform only the landing, whereas the subjects in our study jumped immediately after landing, which is arguably more specific to sport maneuvers. Weinhandl et al. (27) suggested that reduced joint angles during unilateral landings were an adjustment to improve stability when landing with a reduced base of support. However, this stiff landing likely increases the chances of knee shearing forces with a shifting of the femur and tibia. Our data indicate that reduced hip and knee flexion in combination with the knee valgus may be a greater risk factor than absolute knee valgus alone. Although a high incidence of ACL injury occurs during unilateral landings, higher knee valgus, typically suggested as an increased ACL risk factor, was found with the bilateral jumps along with more hip and knee flexion, which indicated a softer drop jump. Thus, it is possible that a greater knee valgus position without consideration of the hip and knee flexion may not be a significant predictor of ACL injury. In contrast, a 40-cm drop was used by Pappas et al. (25) who found greater knee valgus motion with reduced knee flexion during unilateral landings compared with bilateral landings. Dropping from the higher height in this previous study may have produced greater lack of frontal plane control and knee collapse into a valgus position. Pappas et al. (25) suggested that the higher incidence of ACL noncontact injury occurring during unilateral landings (23) is related to a higher demand to control the landing with the reduced musculature from using 1 leg. These data reveal a need to develop training programs intended to improve unilateral strength, power, and landing mechanics.
Finally, it is important to note several limitations to this study. Ground reaction forces, muscle activation, and loads on the ACL were not measured. An increase in trunk flexion has been shown to reduce quadriceps activity and ground reaction forces (4) but was also unknown in the study. In addition, changes in hip and knee strength were not measured in isolation, and thus, the association between joint strength and kinematics could not be analyzed. Further research is needed to investigate the effect of resistance training on these factors related to ACL injury.
Inclusion of weight-bearing free weight resistance training without other modes of training appears to produce a softer landing with increased knee flexion that would reduce the risk of ACL injury in female recreational athletes. Other modes of training such as plyometrics may be necessary to further reduce the risk of ACL injury by producing a softer landing with increased hip flexion. Other modes of training may also be required in combination with resistance training to reduce knee valgus during a drop jump. In comparison with the bilateral drop jumps, the lower hip and knee flexion angles produced by the unilateral drop jumps indicate a need to increase the emphasis on unilateral training. The increased emphasis on unilateral training during the second half of the 8-week training period did not alter the unilateral drop jump mechanics, which indicate a need for further unilateral strength improvement. Strength training longer than 8 weeks with strength gains above those found in this study may be necessary to increase hip flexion and reduce knee valgus when strength training is the only mode of training.
The authors of this study thank the Physical Fitness and Wellness faculty at the Texas State University for assisting with the study. This study was supported by a grant from the Research Enhancement Program at the Texas State University.
1. Barratta R, Solomonow M, Zhou BH, Muscular coactivation. The role of the antagonist musculature in maintaining knee stability. Am J Sports Med 16: 113–122, 1988.
2. Bell AL, Pedersen DR, Brand RA. A comparison of the accuracy of several hip center location prediction methods. J Biomech 23: 617–621, 1990.
3. Blackburn JT, Padua DA. Influence of trunk flexion on hip and knee joint kinematics during a controlled drop landing. Clin Biomech 23: 313–319, 2008.
4. Blackburn JT, Padua DA. Sagittal-plane trunk position, landing forces, and quadriceps electromyographic activity. J Athletic Train 44: 174–179, 2009.
5. Decker MJ, Torry MR, Wyland DJ, Sterett WI, Steadman JR. Gender differences in lower extremity kinematics, kinetics, and energy absorption during landing. Clin Biomech 18: 662–669, 2003.
6. Hanson AM, Padua DA, Blackburn JT, Prentice WE, Hirth CJ. Muscle activation during side-step cutting maneuvers in male and female soccer athletes. J Athletic Train 43: 133–143, 2008.
7. Heinert BL, Kernozek TW, Greany JF, Fater DC. Hip abductor weakness and lower extremity kinematics during running. J Sport Rehabil 17: 243–256, 2008.
8. Herman DC, Weinhold PS, Guskiewicz KM, Garrett We, Yu B, Padua DA. The effects of strength training on the lower extremity biomechanics of female recreational athletes during a stop-jump task. Am J Sports Med 36: 733–740, 2008.
9. Hewett TE, Lindenfeld TN, Riccobene JV, Noyes FR. The effect of neuromuscular training on the incidence of knee injury in female athletes: A prospective study. Am J Sports Med 27: 699–706, 1999.
10. Hewett TE, Meyer GD, Ford KR, Heidt RS, Colosimo AJ, McLean SG, Bogert AJ, Paterno MV, Succop P. Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior crutiate ligament injury risk in female athletes: A prospective study. Am J Sports Med 33: 492–501, 2005.
11. Hewett TE, Stroupe AL, Nance TA, Noyes FR. Plyometric training in female athletes. Decreased impact forces and increased hamstring torques. Am J Sports Med 24: 765–773, 1996.
12. Jacobs C, Mattacola C. Sex differences in eccentric hip-abductor strength and knee-joint kinematics when landing from a jump. J Sport Rehabil 14: 346–355, 2005.
13. Jacobs C, Uhl T, Mattacola C, Shapiro R, Rayens W. Hip abductor function and lower extremity landing kinematics: Sex differences. J Athletic Train 42: 76–83, 2007.
14. Krause DA, Jacobs RS, Pilger KE, Sather BR, Sibunka SP, Hollman JH. Electromyographic analysis of the gluteus medius in five weight-bearing exercises. J Strength Cond Res 23: 2689–2694, 2009.
15. Lephart SM, Abt JP, Ferris CM, Sell TC, Nagai T, Myers JB, Irrgang JJ. Neuromuscular and biomechanical characteristic changes in high school athletes: A plyometric versus basic resistance program. Br J Sports Med 39: 932–938, 2005.
16. Lephart SM, Ferris CM, Riemann BL, Myers JB, Fu FH. Gender differences in strength and lower extremity kinematics during landing. Clin Orthop Relat Res 401:162–169, 2002.
17. Lim B, Lee YS, Kim JG, An KO, Yoo J, Kwon YH. Effects of sports injury prevention training on the biomechanical risk factors of anterior cruciate ligament injury in high school female basketball players. Am J Sports Med 37: 1728–1734, 2009.
18. Lloyd DG, Buchanan TS. Strategies of muscular support of varus and valgus isometric loads at the human knee. J Biomech 34: 1257–1267, 2001.
19. Mascal CL, Landel R, Powers C. Management of patellofemoral pain targeting hip, pelvis, and trunk muscle function: 2 case reports. J Ortho Sports Phys Ther 33: 642–660, 2003.
20. McCurdy K, O'Kelley E, Kutz M, Langford G, Ernest J, Torres M. Comparison of lower extremity EMG between the 2-leg squat and modified single-leg squat in female athletes. J Sport Rehabil 19: 57–70, 2010.
21. McNair P, Prapavessis H, Callender K. Decreasing landing forces: Effect of instruction. Br J Sports Med 34: 293–296, 2000.
22. Meyer GD, Ford KR, Brent JL, Hewett TE. The effects of plyometric vs. dynamic stabilization and balance training on power, balance, and landing force in female athletes. J Strength Cond Res 20: 345–353, 2006.
23. Olsen O, Myklbust G, Engebretsen L, Bahr R. Injury mechanisms for anterior cruciate ligament injuries in team handball. Am J sports Med 32: 1002–1012, 2004.
24. Padua DA, Marshall SW. Evidence supporting acl-injury-prevention exercise programs: A review of the literature. Athletic Ther Today 11: 11–23, 2006.
25. Pappas E, Hagins M, Sheikhzadeh A, Nordin M, Rose D. Biomechanical differences between unilateral and bilateral landings from a jump: Gender differences. Clin J Sport Med 17: 263–268, 2007.
26. Salsi Y, Kentel BB, Heycan C, Akin S, Korkusuz F. Comparison of landing maneuvers between male and female college volleyball players. Clin Biomech 19: 622–628, 2004.
27. Weinhandl JT, Joshi M, O'Connor KM. Gender comparisons between unilateral and bilateral landings. J Appl Biomech 26: 444–453, 2010.
28. Youdas JW, Loder EF, Moldenhauer JL, Paulsen CR, Hollman JH. Hip-abductor muscle performance in participants after 45 seconds of resisted sidestepping using an elastic band. J Sport Rehabil 15: 1–11, 2006.
29. Zeller BL, McCrory JL, Kibler WB, Uhl TL. Differences in kinematics and electromyographic activity between men and women during the single-legged squat. Am J Sports Med 31: 449–456, 2003.