Female athletes are more likely to suffer a noncontact anterior cruciate ligament (ACL) injury during sports compared with male athletes (3). Despite a considerable amount of research focused on understanding the noncontact ACL injury mechanism and determining the associated risk factors, the overall incidence of noncontact ACL injuries in female athletes has remained high (1,2). As part of this research focus, numerous studies have investigated lower extremity mechanics in females during sport-specific tasks (e.g., landing and cutting) to better understand modifiable factors that may manifest within the noncontact ACL injury mechanism (10,15,23,26,29). These studies suggest that females exhibit movement and loading patterns that place them at a greater risk for ACL injury. In addition, these findings are supported by evidence that some of these patterns, such as greater peak external knee abduction moments and peak abduction angles, prospectively predict ACL injury risk in female athletes (15). Despite the considerable amount of evidence that implicate movement and loading patterns as part of the noncontact ACL injury mechanism, it is not well understood how other aspects such as central processing, environment, and physical abilities influence biomechanical patterns during movement tasks.
Based on the fact that noncontact ACL injuries occur primarily during competition where athletes have to process and respond to information under time-critical situations, researchers have designed experimental paradigms that use unanticipated tasks to study lower extremity kinematics and kinetics because these scenarios may provide better insight into movement and loading patterns that athletes would encounter during competition (6,8). When compared with anticipated tasks, lower extremity biomechanics during unanticipated tasks are consistent with kinematic and kinetic profiles known to increase the risk of ACL injury (6,8). For example, knee valgus and internal rotation moments during the stance phase of unanticipated side-step cutting, jumping, and landing tasks are greater than during similar tasks under anticipated conditions (6,8). The use of unanticipated landing paradigms may therefore offer better insights into possible “worst-case scenarios” of lower extremity movement and loading patterns that one could expect to experience during a practice or game situation (22).
In addition, it is important to gain an understanding of the influence of trainable physical abilities (e.g., muscular strength) on the lower extremity biomechanics during movement tasks, especially those tasks that are performed under time-critical situations. For example, it could be posited that hip muscle weakness could exacerbate deleterious landing kinematics and kinetics during unanticipated movement tasks as individuals would have less time to adequately control the joint motions. Indeed, practitioners and researchers commonly emphasize the role of the hip musculature in relation to lower extremity movement and loading patterns (4,5,7,11,16,17,19,25,27,30). For example, Lawrence et al. (20) found that during an anticipated single-leg drop landing, individuals with greater strength of the hip external rotator muscles better attenuated vertical ground reaction forces and exhibited smaller knee valgus moments than individuals with lower hip external rotator strength. Furthermore, in another study, individuals who demonstrated improvements in hip muscle performance after a targeted hip strengthening intervention exhibited positive changes in lower extremity kinetics that were consistent with a lower risk for ACL injury (30). Moreover, hip external rotator strength appears to be a significant predictor of the incidence of lower extremity injury risk during an entire athletic season in both males and females (21). Although these studies provide collective support for the hypothesis that hip muscle strength affects the dynamic control of the lower extremity during athletic movement tasks, it is currently not known to what extent hip strength affects dynamic kinematic and kinetic control during unanticipated single-leg landing and cutting tasks.
Understanding the association between hip strength and lower extremity kinematics and kinetics is important because of the implications for biomechanical risk factors linked to the noncontact ACL injury mechanism. Practitioners could use this information to develop hip strengthening interventions that translate to improvements in landing mechanics during time-critical game-like situations. The purpose of this study was to determine the influence of hip muscle strength on lower extremity kinematics and kinetics during unanticipated single-leg landing task. We hypothesized that females with less hip abductor and external rotator strength will demonstrate movement and loading patterns that are consistent with greater risk for ACL injury.
Experimental Approach to the Problem
To study the association between hip strength and dynamic lower extremity kinematics and kinetics control during unanticipated single-leg landing task, female soccer players reported to a biomechanics laboratory for movement testing and strength assessment. Maximal isometric hip abduction and external rotation torque were collected and used as the independent variables for the study. Dependent variables measured were the calculated 3-dimensional lower extremity kinematics and kinetics during single-leg landing and cutting tasks. Statistical correlation coefficients were then used to determine the associations between the independent variables (i.e., hip abduction and external rotation torque) and the dependent variables (i.e., hip and knee excursion and peak hip and knee internal joint moments in all planes).
An a priori power analysis (G*power software) revealed that a sample of 19 subjects (age range, 18–22 years) would be sufficient to demonstrate a power 0.80, at the predetermined alpha level of 0.05 and with a moderate effect size of 0.6 (12). Twenty-three female National Collegiate Athletic Association division I soccer players were thus recruited for this study. The average (mean ± SD) age, height, and weight of the participants were 19.4 ± 0.8 years, 167.9 ± 5.0 cm, and 61.0 ± 4.0 kg, respectively. All participants were healthy with no history of musculoskeletal, neurologic, or cardiovascular conditions that would prevent full participation in the testing protocol. All participants completed a health history questionnaire to report injuries that occurred during previous athletic seasons. Athletes were excluded from the study if a hip, knee, or ankle injury was reported during the previous season as this may influence landing mechanics. Before the participation in the study, all of the testing procedures were explained, and written informed consent was obtained. All procedures of this study were performed in accordance with approval from the local university's institutional review board (IRB protocol # HR-2437).
Hip Strength Measures
Maximal isometric hip strength was tested for the hip abductor and external rotator muscle groups using a hand-held dynamometer (Lafayette Manual Muscle Tester Model 01163; Lafayette Instrument Company, Lafayette, IN, USA). The hip abductor muscle group was tested in the side-lying position with the test hip placed in the neutral position, which was defined as 0° of rotation in any plane (Figure 1A). The bottom leg was flexed slightly at the knee and hip to assist with maintaining stability during the test. Pillows were placed between the legs of the subject to maintain a neutral position of the hip in the frontal plane during the testing. The frontal plane hip angle was measured using a standard long-arm goniometer (Sammons Preston, Patterson Medical, Warrenville, IL, USA) to ensure that the hip was in neutral rotation during the test. The hand-held dynamometer was placed 1 inch proximal to the lateral epicondyle of the femur in a perpendicular position to the thigh and was secured to the leg using a custom foam cutout and canvas strap. The strap was secured around the undersurface of the athletic table to prevent slippage during testing. The dynamometer was stabilized by a hand of the examiner to prevent tilt that could lead to nonperpendicular force production. The subject was instructed to press the leg upward toward the ceiling without rotating the pelvis or limb. The subject was instructed to press upward with maximal effort, but to avoid a rapid or jerky movement, in order to slowly build to maximal effort over a 5-second period.
The hip external rotators were tested in a seated position with the hip placed in neutral (0°) frontal and transverse plane rotation (Figure 1B). The dynamometer was placed 1 inch proximal to the medial malleolus of the ankle and secured to the leg using a custom foam pad and canvas strap that was attached to an athletic table. The subject was instructed to sit upright with the arms crossed and hands on each contralateral thigh to prevent flexing the hips or leaning backward during the testing. The subject was instructed to rotate the leg inward while maintaining the thigh in contact with the table. The subject was instructed to press into the strap, but to avoid a rapid or jerky movement, to slowly build to maximal effort over a 5-second period. Maximal hip abductor and external rotator isometric torques were calculated as the product of the respective segment length (i.e., fixed external moment arm) and the isometric force measured with the hand-held dynamometer and were then subsequently normalized to individual body weight (newton meters per kilogram).
Unanticipated Single-Leg Landing and Cutting Tasks
Subjects were required to perform 3 different types of unanticipated single-leg landing tasks that consisted of either a: (a) single-leg land and hold (b) single-leg land and side cut, or (c) single-leg land and forward run. Each subject performed between 3–5 successful trials of each unanticipated single-leg landing and cutting task. Before performing the unanticipated tasks, the height and distance of the box from the force plates were normalized for each subject. The box height was normalized by having each subject first perform 5 maximum vertical jumps while in a position with the hands on the hips and elbows bowed out (i.e., akimbo). An x-y plot of the right posterior superior iliac spine (PSIS) marker position vs. time from the third vertical jump trial was created using Vicon Nexus software 1.8.2 (Vicon Motion Systems Ltd., Oxford, UK). The distance between the initial position of the right PSIS marker during standing and the position of the marker at the maximum vertical jump height was used to normalize the box height to each subject. The rationale for this normalization was that this height would represent the maximum height an individual athlete would land from during competition. The distance of the box from the force plate was normalized to the subject's maximal single-leg jump distance, which was determined by having the subject stand on the right lower extremity in the middle of the force plate and perform a forward single-leg jump onto the left lower extremity. The distance from the edge of the force plate to the front of the athletic shoe of the subject was measured. The front edge of the box was placed at this distance for the unanticipated single-leg landing and cutting tasks.
All single-leg landing tasks required subjects to jump off the box and land on the force plate with 1 leg and stabilize their body during the landing without touching the opposite foot to the ground or taking a step. The single-leg land-and-hold task required subjects to jump off the box and stick the landing while establishing stability as quick as possible. The land-and-side cut task required subjects jump off of the box, land on the force plate, and immediately perform a 90° lateral cut. The single-leg land-and-forward run task required the subject to jump off of the box, land on the force plate, and then immediately run forward after the landing. The subjects were instructed to perform the running and cutting tasks without hesitation after landing on the plate. The tasks were performed randomly with the use of a custom LabVIEW program (National Instruments Corp., Austin, TX, USA, 2013) that was interfaced with a pressure switch mat placed on the top of the box. Once participants broke contact with the switch mat after jumping off, the program displayed a red light (single-leg land), right arrow (land and side cut), or forward arrow (land and forward run) on a large TV screen in front of the subject. The take-off leg for all unanticipated tasks was nondominant leg, such that the landing leg was always the dominant leg, which was defined as the leg with which one would kick a ball (Figure 2).
A 14-camera motion analysis system (Vicon Motion Systems Ltd., Oxford, UK) was used to record kinematic data at a rate of 120 Hz. Kinetic data were collected with an AMTI force plate (AMTI Corp., Watertown, MA, USA) at 960 Hz. The 3-dimensional positions of 23 retroreflective markers were recorded during each task. These markers were attached to the anatomical landmarks of C7 spinous process, T10 spinous processes, sternum, and bilaterally over the PSIS, anterior superior iliac spine (ASIS), iliac crests, greater trochanters, medial and lateral knee joint lines, medial and lateral malleoli, and first and fifth metatarsal heads. Marker clusters, consisting of 4 markers attached to a plastic plate fixed to a Velcro strap, were also attached to the bilateral thighs and shanks (13). In addition, clusters, of 3 markers attached to a plastic plate, were fixed directly to both heels (13). A standing trial in a static position was used to individualize marker position and calculate joint centers and segment positions for each individual subject during all subsequent movement testing.
A fourth-order low-pass Butterworth filter with a cutoff frequency of 12 Hz was used to smooth the kinematic and kinetic data in Visual 3D (C-Motion, Inc., Rockville, MD, USA) (6,8). A local coordinate system was then used to define the model segments for the pelvis, thigh, shank, and foot based on marker position over the anatomical landmarks on the segments. A joint coordinate system approach was used to determine the angles of the hip, knee, and ankle (9,14). An inverse dynamics approach was used to calculate the net internal joint moments (in newtons per meter) using the ground reaction force data and segmental kinematic data (18,28). Net internal joint moments were calculated in the distal relative to proximal segments coordinate system.
The variables of interest were the 3-dimensional hip and knee joint angle excursions and peak joint moments during the landing phase of each unanticipated task. The landing phase was defined as the time period between the point of initial contact and the peak knee flexion angle. Initial ground contact was defined as when the vertical ground reaction force vector was greater than 30 N. Joint excursion was defined as the difference between the joint angle at initial contact and the maximum joint angle during the landing phase. For simplification, all data are represented from the perspective of a right lower extremity with the direction of the data consistent with the right-hand rule.
Initially, the assumptions of parametric statistical tests were checked through the evaluation of Q-Q plots, the Shapiro-Wilk test for normality, and standardized residual plots for homoscedasticity for all variables. All data were determined to meet the assumptions for parametric testing. Next, Pearson's correlation coefficients were calculated to determine the associations between the independent variables (hip abduction and external rotation torque) and the dependent variables (hip and knee excursion and peak hip and knee moments in all planes). Statistical significance was set a priori at α = 0.05. All statistical analyses were performed using SPSS (IBM, Chicago, IL, USA).
Before the correlational analysis, a Multivariate Analysis of Variance (MANOVA) was performed to determine if mean differences exist on the dependent variables among the 3 unanticipated tasks (i.e., land-and-hold task; land-and-cut task; land-and-forward run task). The results demonstrated no significant difference between the tasks for any of the dependent variables (F = 1.095, p = 0.376). Therefore, it was decided that the mean values of the kinematic and kinetic variables from each of the 3 tasks should be combined for final analysis (Table 1). The values presented in Table 1 represent the 3-task averages. The ensemble average for female athletes hip and knee moments from initial contact to maximum knee flexion (i.e., stance phase) during a single-leg unanticipated land-and-cut task is presented to demonstrate the overall kinetic pattern at the hip and knee during landing (Figures 3A, 4A, 5A, B and 6A, B). Additionally, ensemble average transverse plane kinematic patterns at the hip and knee are also presented for clarity (Figures 3B, 4B).
Significant correlations were found between hip external rotator strength and transverse plane joint moments at the hip and knee (Table 2). Specifically, hip external rotator strength was significantly correlated with the peak hip external rotation moment and peak knee internal rotation moment. Frontal plane hip excursion and transverse plane knee excursion were also significantly associated with hip external rotator strength (Table 2). A statistical trend was also present for the correlation between hip external rotator strength and peak hip abduction moment (Table 2). Hip abductor strength was not significantly correlated with any other biomechanical variables (Table 2).
The purpose of this study was to determine the association between hip strength and lower extremity kinematics and kinetics during unanticipated single-leg landing and cutting tasks in female athletes. The most important findings of this study were the associations between hip external rotator strength and hip and knee kinetics during unanticipated single-leg landing and cutting tasks. Specifically, female soccer players with greater hip external rotator strength demonstrated greater peak hip external rotation and abduction moments and peak knee internal rotation moments during landing. In addition, females with greater hip external rotator strength demonstrated greater hip frontal plane excursion and knee transverse plane excursion during unanticipated movement tasks. To our surprise, this study did not reveal any associations between hip abductor strength and any of the biomechanical variables. Regardless, these results partially support our hypothesis that greater hip strength would be associated with better lower extremity kinematic and kinetics during unanticipated single-leg landing and cutting tasks. Therefore, the practical importance of the results, which link hip external rotator strength with hip and knee kinetics during unanticipated single-leg landing and cutting, provides support for the role of hip strength in the dynamic control of the lower extremity during unanticipated landing tasks.
The results of this study showed a moderate correlation (r = −0.48) between hip external rotator strength and peak hip external rotation moments during unanticipated single-leg landing and cutting tasks in female athletes. Given that hip external rotation moments are negative by convention, the correlation indicates that greater hip external rotation moments are associated with greater hip external rotator strength. The association between hip external rotator strength and hip external moments suggests that greater hip strength may promote better dynamic control of the femur and/or pelvis during the landing phase of an unanticipated task. The hip kinematic and kinetic patterns in the transverse plane were occurring in opposite directions, which indicated that hip internal rotation motions were being decelerated by external rotation moments, thereby controlling a rotational aspect of the purported deleterious dynamic valgus (15). Lawrence et al. (20) also showed an association between hip external rotator strength and better dynamic lower extremity control during single-leg landings. These authors demonstrated that females with greater hip external strength experienced smaller ground reaction forces and mean external knee valgus moments. They concluded that weak hip musculature may lead to increased risk of deleterious landing mechanics consistent with noncontact ACL injury (20). It therefore seems rational to argue that female athletes with greater hip external rotator strength are able to better control aspects of dynamic valgus during single-leg unanticipated landing tasks, thereby lessening the risk of noncontact ACL injury.
A moderate correlation (r = 0.42) between hip external rotator strength and knee internal rotation moments during unanticipated landing and cutting tasks was also found. Additionally, a moderate negative correlation (r = −.56) was also found between hip external rotator strength and transverse plane knee excursion. Our transverse plane knee moment findings are consistent with previous investigations that used a similar unanticipated movement task (6). Similar to our observations at the hip joint, the mean transverse kinematic pattern of the knee indicates that females internally rotate at initial contact and then rapidly stabilize the knee in a near neutral transverse plane position. Our findings indicate that the knee and hip were both in an externally rotated position at initial contact, which supports the fact that these female athletes landed with the lower extremity in neutral alignment in the transverse plane. The neutral lower extremity alignment at initial contact and the rapid stabilization of the knee in the transverse plane after initial contact support that hip external rotator strength may also play a role in dynamic transverse plane control at the knee during unanticipated landing and cutting tasks.
Based on the fact that the correlations in this study were between hip external rotation strength and hip and knee kinetic variables, we believe that the gluteus maximus was predominantly responsible for the dynamic control of the unanticipated landing and cutting tasks. The hip sagittal plane kinetic pattern indicates a strong hip extension moment during the landing phase, and the hip transverse plane kinetic pattern resembles a similar shape to the sagittal plane moment pattern. The gluteus maximus muscle exhibits the greatest force generating capacity of all hip muscles and is commonly recruited during tasks that require deceleration of the center of mass or propulsion of the body mass forward and upward such as during walking and vertical jumping (24,32). Moreover, the gluteus maximus is a strong hip external rotator, and we therefore postulate that its role in hip external rotator torque production is the reason that hip and knee kinetics during an unanticipated landing and cutting task were most closely associated with this measure. Although muscle activity was not quantified in this study and the exact mechanism in which the gluteus maximus may control aspects of dynamic valgus is not fully understood, it seems that the hip musculature does play an important role in dynamic control and should be addressed in training and rehabilitation programs for athletes.
This study also found that females with greater hip external rotator strength exhibited greater frontal plane hip excursion into the direction of adduction (i.e., dynamic valgus) during an unanticipated single-leg land-and-cut task. The current results are similar to another research group who found significant correlation between hip external rotation strength and frontal plane knee excursion (i.e., knee valgus) during a drop jump landing (5). Bandholm et al. (5) suggest that, over time, repeatedly landing in greater knee valgus positions leads to movement pattern “adaptation” that is related to gains in hip strength because of the repeated activation of the hip external rotators (i.e., gluteus maximus) as part of this control strategy. In this study, the contralateral pelvis was elevated at initial contact (i.e., relative hip abduction) and moved into a dropped position (i.e., relative hip adduction) throughout the landing phase, which results in frontal plane hip excursion in the direction of adduction. The control of this pattern can be explained by the current studies statistical trend between hip external rotator strength and hip abduction moments during the single-leg unanticipated land-and-cut tasks. When the hip frontal plane kinematics and kinetic patterns are considered in combination, it seems that hip frontal plane excursion in the direction of adduction was being controlled by an oppositely directed hip abduction moment. Therefore, the relationship between hip external rotator strength and the frontal plane dynamic control pattern at the hip in this study support the movement “adaptation” previously described by Bandholm et al. (5).
There are several limitations to this study that must be taken into consideration. The sample in this study involved high-level female athletes, which may limit the generalizability of the results to other populations. In addition, the results from a cross-sectional study with correlational analysis cannot be used to determine causation between measures of hip strength and landing mechanics. Future prospective and longitudinal research could provide data that establishes stronger links between hip strength and landing mechanics (30). The use of electromyography to quantify muscle activation patterns may also provide valuable information into the neuromuscular strategies used during a single-leg unanticipated task (16). Additionally, hip abductor strength was tested isometrically in an open chain position, whereas the tasks of interest were closed chain movements. Perhaps, testing the hip abductors and external rotators in a closed chain position would provide a more functional approach to assessing muscle strength for landing tasks (31). Finally, we acknowledge the difference between laboratory-based testing and game situations, even if both are unanticipated.
This study demonstrates that greater hip external rotator strength was associated with greater peak hip external rotation and abduction moments, greater peak knee internal rotation moments, and greater hip frontal plane excursion during unanticipated single-leg landing tasks. The kinetic and kinematic patterns demonstrated in this study support that hip external rotator strength may play a role in the control of dynamic valgus during unanticipated landing and cutting. Therefore, practitioners should consider implementing hip muscle strengthening into training programs for athletes who participate in sports that require landing and cutting.
The authors would like to acknowledge support through the National Center for Advancing Translational Sciences, National Institutes of Health, Grant Number 8UL1TR000055.
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Keywords:Copyright © 2016 by the National Strength & Conditioning Association.
biomechanics; hip strength; dynamic valgus; unanticipated tasks; ACL injury