Prevention of noncontact anterior cruciate ligament (ACL) injury in women has received much research attention with reports showing higher injury rates in comparison with men (1). Although several factors show a relationship with an increased risk of ACL injury in female athletes (23,25), strength is believed to be an important predictor (14,15) and a focus of improvement in training programs. Strength may affect injury risk through the association with neuromuscular activation patterns, joint stiffness, antagonist muscle balance, ground reaction forces, and joint mechanics. Although strength improvement is accepted as an important component of a training program to minimize the risk of ACL injury, the type of resistance training that effectively reduces the rate of this injury is not clearly determined.
Research suggests that joint mechanics during landing is related to noncontact ACL injury rates in female athletes (16). Several studies have analyzed the relationship between strength and joint mechanics during landing maneuvers typically implicated in noncontact ACL injury (12,13,17,34), with the majority of these studies demonstrating low-to-moderate relationships using single-joint, isometric measures of strength (18,21,27,30,34,35). In addition, these measures of strength are most commonly used as a criterion to identify the risk of ACL injury during landing in women and used during training to prevent knee injury. However, athletes typically train using multi-joint, free-weight exercises such as squats for injury prevention and improved performance to achieve specificity. Currently, the association of squat strength and landing mechanics is not well understood. These exercises may demonstrate a stronger relationship with joint mechanics that occur during weight-bearing activity than various types of single-joint and non–weight-bearing strength measures. With the increasing understanding that many athletic maneuvers are performed from a unilateral stance and that many noncontact ACL injuries occur during a unilateral stance, a comparison of the correlations between unilateral and bilateral squat strength and landing mechanics is needed. A higher correlation would signify a need to emphasize specific types of strength training that may improve joint mechanics and reduce injury. This contention was supported by Herman et al. (13) who found no change in landing mechanics in women after strength training with single-joint exercises.
Noncontact anterior cruciate ligament injuries typically occur during the support phase while the muscle is contracting eccentrically. This would indicate that eccentric strength may be an important determinant of joint mechanics during landing, but few studies have analyzed the relationship between eccentric strength and joint mechanics (2,19) and jump performance (2). Although eccentric strength is primarily measured non–weight bearing and in joint isolation, it may demonstrate a high relationship with joint mechanics during landing with both using the same type of muscle contraction.
The relationship between strength and joint mechanics is not clearly understood, in part, due to different measures of strength and conditions used to measure joint mechanics across different research studies (7). Few studies have analyzed the relationship between strength measured with multijoint exercises like the squat and landing performance (31). To the best of our knowledge, a comparison of different multijoint weight-bearing strength measures and joint mechanics is yet to be reported. In addition, variations in intensity (unilateral vs. bilateral, drop height, body weight squat, and stepping down from steps) used to analyze joint mechanics likely affect the association with various measures of strength (2,5,9,21) and requires further investigation. Research has shown that more noncontact ACL injuries occur during unilateral landings (26). With the use of half of the lower-extremity musculature to support a unilateral landing, it would seem logical that strength would be a contributing factor predicting mechanical control due to the challenging nature of the task.
With limited data directly comparing the relationship between various measures of strength on unilateral and bilateral landings in the same investigation, it is difficult to conclude the magnitude of the relationship between strength and joint mechanics related to ACL injuries. A direct comparison of the relationship will lend evidence for strength assessment predicting hip and knee mechanics that may be used to screen for increased risk of knee injury and improve exercise selection when designing knee prevention training programs. Therefore, the purpose of this study was to compare the relationship among various single-joint, non–weight-bearing and multijoint weight-bearing exercises, and hip and knee mechanics during unilateral and bilateral drop jumps. We hypothesized that multijoint weight-bearing strength would produce the strongest correlations, and the bilateral and unilateral squat would produce the highest correlations on the bilateral and unilateral drop jumps, respectively. We also hypothesized that unilateral landings would tend to show higher correlations with strength and eccentric strength would demonstrate higher correlations than isometric strength with knee excursion.
Experimental Approach to the Problem
Women tend to perform weight-bearing maneuvers with hip and knee patterns that increase their risk of ACL injury (23). Strength training is commonly accepted as a method to improve landing mechanics associated with the risk of ACL injury in women; however, the type of strength that is most related to these landing mechanics is currently not clearly understood. As a result, this is a cross-sectional design to analyze if differences exist in the association between several measures of strength and hip and knee landing mechanics considered to determine the risk of ACL injury in women. Determining differences will provide support for specific exercise selection during strength assessment and training for the prevention of ACL injury in women. Hip adduction, hip and knee internal rotation, knee valgus, and hip and knee flexion have been investigated to determine the patterns most associated with ACL injury (24). As a result, these motions were included in the analysis to compare the relationships with different measures of strength in female participants. Unilateral and bilateral landings produce noncontact ACL injury, however, different lower-extremity patterns occur with unilateral and bilateral landings (32), thus, both were included in the design. To analyze the effect of specificity between strength and landing mechanics, the number of joints and types of contraction were considered as a basis for the variables included. Single- and multijoint, isometric and isokinetic eccentric strength, along with weight-bearing and non–weight-bearing strength measures were included to determine whether the type of strength affects the relationship with the selected movement patterns within the same investigation. This is the first known study to compare isometric, eccentric, isokinetic, and 2 types of squat strength in the same study. To further analyze specificity, 2 types of squat measures were included in the design. With differences in neuromuscular demands previously found between the bilateral free-weight squat (BS) and a modified single-leg squat (MSLS) (22), dictated by the base of support producing differences in stabilization within the planes, it is possible that having a similar squat and landing stance will produce the strongest correlations.
Twenty six healthy female subjects with previous high school athletic experience (height, 165.1 ± 7.01 cm; mass, 60.91 ± 7.14 kg; age, 20.9 ± 1.62 years) completed this study. The subjects were surveyed for exclusion criteria that included any current or previous injuries and physical conditions that would have affected the strength and jump performance. The subjects were recreationally active in sport before the beginning of the study. All subjects had 2–10 years of participation in athletic competition in various sports (soccer, tennis, basketball, volleyball, and softball), whereas 2 were current collegiate athletes. All subjects signed an inform consent document after the investigation was approved by the university's Institutional Review Board for use of human subjects in research.
Drop Jump Test
The subjects reported to the laboratory to be familiarized with the jump and strength procedures and to record the age, height, and weight. Practice trials took place on the drop jumps and strength exercises. The subjects were instructed to refrain from strength training 72 hours before the start of the study, get 8 hours of sleep, and maintain a normal diet and hydration. All tests took place at a similar time of the day. Drop jumps, single-joint strength, BS, and the MSLS were assessed on separate days with 48 hours between strength measures.
After a general and dynamic warm-up and light stretching, the drop jumps were completed in the first session. The subjects kept the hands on the hips and stepped off of a box (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. On landing, the subjects jumped for maximum height. The stance leg used during a kick was analyzed during the unilateral and bilateral jumps that were completed in random order. Three trials for each type of jump were performed with 30 seconds of rest between trials and a 2-minute rest period between each test. To eliminate potential learning effect across trials, 2–4 practice trials took place.
Three-dimensional hip and knee motion 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 stretch tape followed on top with athletic tape. 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 axis 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. The right and left anterior superior iliac spine were landmarks to determine the hip joint center using the method by Bell et al. (3). Euler angle sequence 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 lowpass fourth order Butterworth filter (4). Mean knee valgus, knee rotation and flexion, hip flexion, hip adduction, and rotation angles from the 3 trials were analyzed. To determine the test-retest reliability for these measures, Cronbach's alpha intraclass correlation coefficients (ICCs) were calculated. The ICCs for the mean of 3 trials for these variables ranged from 0.91 to 0.99. Because these ICCs were all very high, these data were considered appropriate for analysis.
Single-Joint Strength Tests
After the motion analysis, 48 hours of rest and a dynamic warm-up, all single-joint strength measures took place in 1 session in random order. Isometric hip extension, external rotation, and abduction strength (lbs) were measured using a handheld dynamometer (Hoggan Health Industries, Inc., West Jordan, UT, USA). The dynamometer was stabilized using a strap that wrapped around it and the laboratory table. Hip extension and abduction were measured with the subject lying prone and on the side, respectively following protocols from previous research (30,34). Hip external rotation procedures were followed from Hollman et al. (17). Straps were also used to stabilize the waist and upper body on all tests. The subjects were instructed to reach max force after 2 seconds and continue for 3 more seconds. Thirty seconds of rest were allowed between 3 trials. Two to 3 minutes of rest took place between each test. Eccentric isokinetic quadriceps and hamstring strength were assessed using the Biodex IV Isokinetic Dynamometer (Biodex Medical Systems, Inc., Shirley, NJ, USA) following the procedures of previous research (9). Three repetitions were completed at 90°·s−1. Isometric knee flexion and extension torque measures were assessed following Stearns et al. (29) using the Biodex IV protocol with the mean of the peak torques recorded across 3 trials. Cronbach's alpha ICCs were also calculated for these variables to determine the test-retest reliability. The ICCs for the mean of 3 trials for these variables ranged from 0.92 to 0.97.
Multijoint Strength Tests
After the single-joint measures and 48 hours of rest, a 3 repetition maximum (3RM) free-weight, BS assessment took place to predict the 1 repetition maximum (1RM). After a general and dynamic warm-up and light stretching, the subjects completed 2 warm-up sets using 40–50% of the subjects estimated 1RM for 3–5 repetitions. An audio feedback monitor (Bigger Faster Stronger, Salt Lake City, UT, USA) was placed across the middle of the thigh and used to determine 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 keep the chest up to eliminate excessive trunk flexion during the squat. The subjects were allowed to use their selected stance width and bar placement. The first trial was approximately 85% of the subject's estimated 1RM. With each successful trial, 10–20% was added to the bar until the subject could not complete 3 repetitions. All subjects completed the 3RM test within 5 trials.
The 3RM MSLS was completed 48 hours after the BS. Using an anterior-posterior stance, the dorsal side of the toes of the trail leg was placed on a 12-inch platform (22). Excessive anterior knee translation (knees anterior to the toes) of the lead leg was subjectively assessed and corrected in practice. During the practice session of the MSLS, the distance between the front edge of the box supporting the trail leg and the lead toes (ranging from 96 to 111 cm) that simulated the BS knee position over the toes was determined for each subject. The same procedures used for the BS were followed to determine the 3RM.
A cross-sectional design was used to analyze the association between different types of strength and landing mechanics. Each variable was also tested for normality to meet the basic assumption for correlation. Kolmogorov-Smirnov tests for normality determined that the distributions for each measured variable did not differ significantly from a normal distribution. The dependent variables in this study were the bilateral and unilateral jump measures, whereas the independent variables were the strength measures. The relationship between the bilateral and unilateral jump measures and the strength measures was determined by Pearson's product-moment correlations. Zero-order correlations were calculated between each dependent and independent variable. For each jump measure that was significantly correlated with both squat and another strength measure, partial (first-order) correlations were calculated between the jump measure and strength measure, controlling for the variability due to squat. Significance was defined as p ≤ 0.05 for each correlation.
Descriptive values for both the strength and jump measures are reported in Table 1. The correlations between the strength measures and each of the bilateral jump measures are reported in Table 2. Knee valgus and hip adduction were the only bilateral jump measures that were significantly correlated with the BS and MSLS and some of the isometric strength measures. The correlations between the strength measures and each of the unilateral jump measures are reported in Table 3. Knee valgus and unilateral hip adduction also revealed a strong correlation with both squat strength measures and low-to-moderate correlations with several of the isometric strength measures. Knee internal rotation during the unilateral landing was also significantly correlated with eccentric knee flexor mean peak torque.
After controlling for the variability due to the bilateral or unilateral squat, partial, first-order correlations were calculated comparing the relationship between knee valgus and hip adduction with the other strength measures (Table 4). During the bilateral jump, isometric knee flexion strength remained significantly related to hip adduction (−0.57), whereas isometric hip abduction strength remained significantly related to knee valgus (−0.43). During the unilateral jump, only isometric hip external rotation strength remained significantly related to knee valgus (−0.41).
Although hip and knee mechanics during unilateral and bilateral landings are considered risk factors for knee injury, knowing the specific types of strength with the strongest association with these motions is essential during assessment and exercise prescription to reduce the risk. Supporting our first hypothesis, the strongest correlations found were between the MSLS and BS and knee valgus during the unilateral and bilateral drop jumps (−0.77 ≤ r ≤ −0.83). These strong correlations indicate that these strength measures could be used as screening assessments to predict movement patterns that are related to an increased risk of ACL injury. From these data, women with higher squat strength will reduce the risk by producing less valgus motion. Although strength training is thought to contribute to reducing the risk of ACL injury when combined with other training methods (14), these data also suggest that the MSLS and BS would be a preferred type of strength exercise used; however, training studies are needed to support this speculation.
The hypothesis that the MSLS would produce stronger correlations on the unilateral drop jump and the BS would correlate best with joint motion on the bilateral drop jump was not supported. Similar moderate and significant correlations were found between these exercises and hip adduction (−0.5 ≤ r ≤ −0.65) and knee valgus excursion during unilateral and bilateral drop jumps. The MSLS, in comparison with the BS, creates a higher demand to control frontal plane motion indicated by producing higher activation in the gluteus medius and hamstrings with a narrow medial-lateral base of support (22). Thus, we expected to find higher correlations between MSLS strength and frontal plane motions. The negative correlations indicate that women, who commonly have higher valgus and hip adduction angles than men during the stance phase of a variety of tasks (20), may produce less hip and knee motion in the frontal plane with higher levels of strength on the MSLS and BS. Practitioners can use this squat strength data as criteria of ACL injury risk with both types of squats producing similar predictive ability of hip and knee mechanics during unilateral and bilateral landings.
Low-to-moderate correlations (−0.25 ≤ r ≤ −0.61) were found between isometric single-joint strength and knee valgus. For these isometric measures, all but hip extension strength was significant on the bilateral drop jump, whereas only hip external rotation strength and knee flexion strength were significantly related to knee valgus on the unilateral drop jump. In addition, isometric strength measured with a multijoint leg press did not significantly predict landing kinematics in a previous study by Carcia et al. (6). These data provide support that squat strength, a multijoint exercise using free-weight resistance, may be the best predictor of landing mechanics. The isometric measures of strength in this study were also non–weight bearing, which may have contributed to the lack of consistent significant relationships with the weight-bearing drop jumps (7). These factors require consideration during the exercise selection in designing injury prevention training programs.
The only significant predictor of all isometric strength measures on hip adduction during the unilateral drop jump was knee flexion strength (−0.49), whereas knee flexion strength (−0.68) and knee extension strength (−0.45) were significant predictors of hip adduction on the bilateral drop jump. Although the quadriceps and hamstrings both have moment arms to resist motion in the frontal plane, it seems that isometric hamstring strength is a more consistent and stronger predictor of frontal plane control. Providing similar results, Wild et al. (33) concluded that women with lower hamstring strength had higher knee abduction angles, lower hip abduction moments, and higher ACL loading at peak anterior-posterior ground reaction forces.
None of the isometric hip strength measures were significant predictors of hip excursion. These data are in agreement with many studies who found low and nonsignificant correlations between isometric hip abduction strength and hip adduction control during landing (5,10,21,27,34). These results may indicate that single-joint strength training may not affect hip and knee mechanics. In support of this conjecture, Herman et al. (13) did not find a significant change in hip adduction or knee valgus after single-joint band training of the hip and knee. In contrast, Wallace et al. (31) showed that BS strength and isometric hip abduction strength were related to hip adduction control. Jacobs et al. (20) and Youdas et al. (36) also showed that fatigue of the hip abductors increased hip adduction. Jacobs et al. (20) compared men and women and found that women had lower hip abduction strength and more hip adduction during landing, whereas Youdas et al. (36) used side stepping to fatigue the hip abductors in a group of healthy women to show an increase in hip adduction. Thus, large reductions in strength through fatigue or subjects with vast differences in strength may reveal a stronger relationship between strength and landing mechanics, and in part, explain differences across studies.
When using BS and MSLS strength as the control in a partial correlation to further determine the relationship between isometric strength and hip and knee excursion, several significant correlations were no longer significant. After controlling for squat strength, hip external rotation strength and knee valgus was the only significant relationship during the unilateral drop jumps. For the bilateral drop jumps, hip abduction strength remained significantly related to knee valgus along with knee flexion strength and hip adduction. These data indicate that knee flexion, hip abduction, and hip external rotation strength may be preferred isometric measures to predict frontal plane control when the BS and MSLS are not possible or used in addition to the BS and MSLS; however, further research is warranted.
The hypothesis that eccentric knee strength would demonstrate stronger correlations with knee control than the isometric measures was only supported between knee internal rotation during the unilateral drop jump and knee flexion mean peak torque (−0.4). As mentioned, some of the isometric strength measures were significantly related to knee valgus, but none predicted knee rotation. During ACL injury while landing, the musculature is commonly contracting eccentrically; thus, we hypothesized finding a significant relationship between eccentric strength and knee motion. Research has shown that women have higher knee internal rotation compared with male counterparts (8). The hamstrings produce knee rotation and are likely capable of minimizing knee internal rotation during landing through an eccentric contraction of the biceps femoris. In support, Fujii et al. (11) found that higher bicep femoris activity was related to lower knee internal rotation measurements only in women during landing. The non–weight-bearing measure of isokinetic eccentric strength with single-joint isolation in this study may have attenuated the association with a weight-bearing landing task. These data indicate that isokinetic eccentric strength has low predictive ability of landing mechanics in women. Determining the relationship between eccentric hamstring strength and knee rotation requires further investigation, as we did not find a consistent and strong relationship on all analyses.
It is unclear why eccentric strength, as well as the other strength measures, was not related to the amount of hip and knee flexion. With evidence that greater knee extension strength is related to greater energy absorption from the knee musculature producing a softer landing (28), it may be possible that greater knee strength reduces the risk of injury without affecting the amount of knee motion (21). To control knee flexion, the quadriceps work eccentrically and the hamstrings eccentrically control hip flexion. It is possible that the bilateral landing from 60 cm was not challenging enough in the sagittal plane. Higher hip and knee flexion angles have been suggested to reduce ground reaction forces producing a soft landing, thereby reducing ACL stress (25). Although less hip and knee motion occurred during the unilateral landing, the subjects may have reduced the sagittal plane demand for strength by producing a stiff landing with greater impact forces and absorbing the load with passive joint structures, which is supported in previous research (32). Landing with less flexion also reduces the contribution of strength for knee and hip control with less resistance torque by reducing the resistance arm at the hip and knee. This unilateral landing strategy is likely preplanned with the anticipation of in-sufficient strength due to the use of 1 leg. This likely compensation may explain why our hypothesis that unilateral landings would demonstrate higher correlations with strength was not supported.
Several limitations of the study require noting. Strength is only 1 factor that may affect mechanics. Feedback from coaches on mechanics, muscle balance, amount and timing of muscle activation, balance and training experience are other factors that could be associated with landing mechanics. Also, it is possible that other eccentric actions, besides knee flexion and extension, could be related to the degree of hip and knee motion (19). The subjects in this study had diverse athletic backgrounds, thus, different results may occur with elite athletes and other populations. Finally, the landing conditions may not have represented the intensity and specific loading conditions that occur during competition.
Frontal plane control during landing is recognized as an important factor related to the risk of ACL injury. The strongest correlations occurred between the BS and MSLS and frontal plane control of the hip and knee across unilateral and bilateral landings. Although the single-joint strength measures were not consistently related to joint mechanics, several low-to-moderate and significant correlations were found for frontal plane control. Knee flexor strength was the most consistent single-joint strength measure related to hip and knee control, which would warrant assessment and improvement with observed weakness. Women with higher levels of strength, particularly squat strength, produced mechanics that tend to reduce the risk of ACL injury. Thus, coaches and trainers should place the highest emphasis on the evaluation and improvement of squat strength in comparison with other strength measures. The stronger correlations found on the BS and MSLS would also indicate that these exercises would have the most impact on hip and knee landing mechanics after training, but this speculation requires longitudinal investigation.
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