Sex differences in lower extremity biomechanics during single leg landings

Biomechanical approaches to understanding the potentially injurious demands of gymnastic-style impact landings

Gymnasts are exposed to a high incidence of impact landings due to the execution of repeated dismount performances. Biomechanical research can help inform recent discussions surrounding a proposed rule change in potentially injurious gymnastic dismounting. The review examines existing understanding of the mechanisms influencing the impact loads incurred in gymnastic-style landings achieved using biomechanical approaches. Laboratory-based and theoretical modelling research of inherent and regulatory mechanisms is appraised. The integration of the existing insights into injury prevention interventions studies is further considered in the appraisals. While laboratory-based studies have traditionally been favoured, the difficulty in controlling and isolating mechanisms of interest has partially restricted the understanding gained. An increase in the use of theoretical approaches has been evident over the past two decades, which has successfully enhanced insight into less readily modified mechanisms. For example, the important contribution of mass compositions and 'tuned' mass coupling responses to impact loading has been evidenced. While theoretical studies have advanced knowledge in impact landing mechanics, restrictions in the availability of laboratory-based input data have suppressed the benefits gained. The advantages of integrating laboratory-based and theoretical approaches in furthering scientific understanding of loading mechanisms have been recognised in the literature. Since a multi-mechanism contribution to impact loading has been evident, a deviation away from studies examining isolated mechanisms may be supported for the future. A further scientific understanding of the use of regulatory mechanisms in alleviating a performer's inherent injury predisposition may subsequently be gained and used to inform potential rule changes in gymnastics. While the use of controlled studies for providing scientific evidence for the effectiveness of gymnastics injury counter measures has been advocated over the past decade, a lack of information based on randomised controlled studies or actual evaluation of counter measures in the field setting has been highlighted. The subsequent integration of insight into biomechanical risk factors of landing with clinical practice interventions has been recently advocated.

Methodological considerations of task and shoe wear on joint energetics during landing

To better understand methodological factors that alter landings strategies, we compared sagittal plane joint energetics during the initial landing phase of drop jumps (DJ) vs. drop landings (DL), and when shod vs. barefoot. Surface electromyography, kinematic and kinetic data were obtained on 10 males and 10 females during five consecutive drop landings and five consecutive drop jumps (0.45m) when shod and when barefoot. Energy absorption was greater in the DJ vs. DL (P=.002), due to increased energy absorption at the hip during the DJ. Joint stiffness/impedance was more affected by shoe condition, where overall stiffness/impedance was greater in shod compared to barefoot conditions (P=.036). Further, hip impedance was greater in shod vs. barefoot for the DL only (via increased peak hip extensor moment in DL), while ankle stiffness was greater in the barefoot vs. shod condition for the DJ only (via decreased joint excursion and increased peak joint moment in DJ vs. DL) (P=.011). DJ and DL place different neuromechanical demands upon the lower extremities, and shoe wear may alter impact forces that modulate stiffness/impedance strategies. The impact of these methodological differences should be considered when comparing landing biomechanics across studies.

Sex differences in proximal control of the knee joint

Following the onset of maturation, female athletes have a significantly higher risk for anterior cruciate ligament (ACL) injury compared with male athletes. While multiple sex differences in lower-extremity neuromuscular control and biomechanics have been identified as potential risk factors for ACL injury in females, the majority of these studies have focused specifically on the knee joint. However, increasing evidence in the literature indicates that lumbo-pelvic (core) control may have a large effect on knee-joint control and injury risk. This review examines the published evidence on the contributions of the trunk and hip to knee-joint control. Specifically, the sex differences in potential proximal controllers of the knee as risk factors for ACL injury are identified and discussed. Sex differences in trunk and hip biomechanics have been identified in all planes of motion (sagittal, coronal and transverse). Essentially, female athletes show greater lateral trunk displacement, altered trunk and hip flexion angles, greater ranges of trunk motion, and increased hip adduction and internal rotation during sport manoeuvres, compared with their male counterparts. These differences may increase the risk of ACL injury among female athletes. Prevention programmes targeted towards trunk and hip neuromuscular control may decrease the risk for ACL injuries.

Foot clearance in walking and running in individuals with ankle instability

BACKGROUND

Foot positioning before heel strike has been attributed to chronic ankle instability injury mechanics, and may play a role in developing and perpetuating chronic ankle instability.

PURPOSE

This study was undertaken to determine if a group of individuals with mechanical instability (MI) or a group with functional instability (FI) of the ankle joint demonstrate less foot-floor clearance and a more inverted and plantar flexed position of the foot during the terminal swing phase of the running and walking cycles when compared with a group of ankle sprain copers who had an injury but no residual instability.

STUDY DESIGN

Controlled laboratory study.

METHODS

Three-dimensional motion analysis was performed on 3 groups (n = 11 male athletes each) differentiated based on ankle injury history and ligamentous laxity during walking and running on a raised platform.

RESULTS

The MI group (14.8° ± 12.0°) demonstrated greater maximum foot external rotation than the FI (3.2° ± 6.0°) and coper groups (2.9° ± 11.0°) (P = .01; η(p) (2) = .25) during running and greater rotation than the coper group during walking (3.3° ± 6.1° vs -4.5° ± 4.1°; P = .03; η(p) (2) = .21). The FI group (6.1° ± 3.2°) had greater plantar flexion at minimum than the MI group (0.1° ± 3.5°) during walking (P = .02; η(p) (2) = .25). Other group differences demonstrated large effect sizes, but not statistical significance, including unstable groups having lower minimum metatarsal height than copers during running.

CONCLUSION

Differences in foot and leg position during terminal swing were observed between MI and FI groups and copers. Greater plantar flexion and lower minimum metatarsal height may increase risk for inadvertent contact and thus episodes of instability.

CLINICAL RELEVANCE

Rehabilitation programs may need to address terminal swing to improve mechanics and avoid potential episodes of giving way at the ankle.

Contribution of knee flexor and extensor strength on sex-specific energy absorption and torsional joint stiffness during drop jumping

CONTEXT

Lower extremity injury often occurs during abrupt deceleration when attempting to change the body's direction. Although sex-specific biomechanics have been implicated in the greater risk of acute knee injury in women than in men, it is unknown if sex differences in thigh strength affect sex-specific energy absorption and torsional joint stiffness patterns.

OBJECTIVE

To determine sex differences in energy absorption patterns and joint stiffnesses of the lower extremity during a drop jump and to determine if these sex differences were predicted by knee extensor and flexor strength.

DESIGN

Cross-sectional study.

SETTING

Laboratory environment.

PATIENTS OR OTHER PARTICIPANTS

Recreationally active, college-aged students (41 women: age  =  22.1 ± 2.9 years, height  =  1.63 ± 0.07 m, mass  =  59.3 ± 8.0 kg; 40 men: age  =  22.4 ± 2.8 years, height  =  1.77 ± 0.1 m, mass  =  80.9 ± 14.1 kg).

INTERVENTION(S)

Participants performed knee flexor and extensor maximal voluntary isometric contractions followed by double-leg drop-jump landings.

MAIN OUTCOME MEASURE(S)

Lower extremity joint energetics (J × N(-1) × m(-1)) and torsional joint stiffnesses (Nm × N(-1) × m(-1) × degrees(-1)) were calculated for the hip, knee, and ankle during the initial landing phase. Body weight was measured in newtons and height was measured in meters. Sex comparisons were made and sex-specific regressions determined if thigh muscle strength (Nm/kg) predicted sagittal-plane landing energetics and stiffnesses.

RESULTS

Women absorbed 69% more knee energy and had 36% less hip torsional stiffness than men. In women, greater knee extensor strength predicted greater knee energy absorption (R(2)  =  0.11, P  =  .04), and greater knee flexor strength predicted greater hip torsional stiffness (R(2)  =  0.12, P  =  .03).

CONCLUSIONS

Sex-specific biomechanics during the deceleration phase of a drop jump revealed that women used a strategy to attempt to decrease system stiffness. Additionally, only female strength values were predictive of landing energetics and stiffnesses. These findings collectively demonstrated that the task may have been more difficult for women, resulting in a different movement strategy among those with different levels of thigh strength to safely complete the task. Future researchers should look at other predictive factors of observed sex differences.

Ankle-dorsiflexion range of motion and landing biomechanics

CONTEXT

A smaller amount of ankle-dorsiflexion displacement during landing is associated with less knee-flexion displacement and greater ground reaction forces, and greater ground reaction forces are associated with greater knee-valgus displacement. Additionally, restricted dorsiflexion range of motion (ROM) is associated with greater knee-valgus displacement during landing and squatting tasks. Because large ground reaction forces and valgus displacement and limited knee-flexion displacement during landing are anterior cruciate ligament (ACL) injury risk factors, dorsiflexion ROM restrictions may be associated with a greater risk of ACL injury. However, it is unclear whether clinical measures of dorsiflexion ROM are associated with landing biomechanics.

OBJECTIVE

To evaluate relationships between dorsiflexion ROM and landing biomechanics.

DESIGN

Descriptive laboratory study.

SETTING

Research laboratory.

PATIENTS OR OTHER PARTICIPANTS

Thirty-five healthy, physically active volunteers.

INTERVENTION(S)

Passive dorsiflexion ROM was assessed under extended-knee and flexed-knee conditions. Landing biomechanics were assessed via an optical motion-capture system interfaced with a force plate.

MAIN OUTCOME MEASURE(S)

Dorsiflexion ROM was measured in degrees using goniometry. Knee-flexion and knee-valgus displacements and vertical and posterior ground reaction forces were calculated during the landing task. Simple correlations were used to evaluate relationships between dorsiflexion ROM and each biomechanical variable.

RESULTS

Significant correlations were noted between extended-knee dorsiflexion ROM and knee-flexion displacement (r  =  0.464, P  =  .029) and vertical (r  =  -0.411, P  =  .014) and posterior (r  =  -0.412, P  =  .014) ground reaction forces. All correlations for flexed-knee dorsiflexion ROM and knee-valgus displacement were nonsignificant.

CONCLUSIONS

Greater dorsiflexion ROM was associated with greater knee-flexion displacement and smaller ground reaction forces during landing, thus inducing a landing posture consistent with reduced ACL injury risk and limiting the forces the lower extremity must absorb. These findings suggest that clinical techniques to increase plantar-flexor extensibility and dorsiflexion ROM may be important additions to ACL injury-prevention programs.

Joint laxity is related to lower extremity energetics during a drop jump landing

PURPOSE

To examine the relationships between anterior knee laxity (AKL), genu recurvatum (GR), and general joint laxity (GJL) with sagittal plane energetics in males and females during a drop jump task.

METHODS

A total of 68 females and 50 males were measured for AKL, GR, and GJL and were instrumented to obtain neuromuscular and biomechanical data on their dominant limb during the initial landing phase of a 45-cm drop jump. Multiple linear regressions determined the extent to which the three joint laxity variables combined to predict hip, knee, and ankle work absorption and stiffness. Associations between joint laxity and joint kinematics, joint kinetics, and muscle activation amplitudes were also investigated to further interpret significant relationships.

RESULTS

Higher AKL and GJL and lower GR combined to predict greater knee work absorption (R2 = 0.210, P = 0.002) and stiffness (R2 = 0.127, P = 0.033) and lower ankle stiffness (R2 = 0.115, P = 0.048) in females. These associations were modulated through greater peak knee extensor moments and flexion angles, lower hamstring activation, and lower ankle extensor moments. In males, joint laxity had little impact on knee energetics, but a significant association was observed between greater GJL and decreased ankle stiffness (R2 = 0.209, P = 0.012), a product of both greater peak ankle flexion and decreased ankle extensor moment.

CONCLUSIONS

Females with greater AKL and GJL and lower GR demonstrated a landing strategy that increased work absorption and stiffness about the knee, whereas females with greater GR demonstrated a landing style that reduced knee work absorption and stiffness. The findings suggest that AKL, GR, and GJL may represent distinct risk factors and support the need to consider more comprehensive laxity profiles as they relate to knee joint function and anterior cruciate ligament injury risk.

The association between lower extremity energy absorption and biomechanical factors related to anterior cruciate ligament injury

BACKGROUND

Greater total energy absorption by the lower extremity musculature during landing may reduce stresses placed on capsuloligamentous tissues with differences in joint contributions to energy absorption potentially affecting anterior cruciate ligament injury risk. However, the relationships between energy absorption and prospectively identified biomechanical factors associated with non-contact anterior cruciate ligament injury have yet to be demonstrated.

METHODS

Sagittal plane total, hip, knee and ankle energy absorption, and peak vertical ground reaction force, anterior tibial shear force, knee flexion and knee valgus angles, and internal hip extension and knee varus moments were measured in 27 individuals (14 females, 13 males) performing double leg jump landings. Correlation coefficients assessed the relationships between energy absorption during three time intervals (initial impact phase, terminal phase, and total landing) and biomechanical factors related to anterior cruciate ligament injury.

FINDINGS

More favorable values of biomechanical factors related to non-contact anterior cruciate ligament injury were associated with: 1) Lesser total (R(2)=0.178-0.558), hip (R(2)=0.229-0.651) and ankle (R(2)=0.280), but greater knee (R(2)=0.147) energy absorption during the initial impact phase; 2) Greater total (R(2)=0.170-0.845), hip (R(2)=0.599), knee (R(2)=0.236-0.834), and ankle (R(2)=0.276) energy absorption during the terminal phase of landing; and 3) Greater knee (R(2)=0.158-0.709), but lesser hip (R(2)=0.309) and ankle (R(2)=0.210-0.319) energy absorption during the total landing period.

INTERPRETATION

These results suggest that biomechanical factors related to anterior cruciate ligament injury are influenced by both the magnitude and timing of lower extremity energy absorption during landing.

Sagittal knee joint kinematics and energetics in response to different landing heights and techniques

Single-leg and double-leg landing techniques are common athletic maneuvers typically performed from various landing heights during intensive sports activities. However, it is still unclear how the knee joint responds in terms of kinematics and energetics to the combined effects of different landing heights and techniques. We hypothesized that the knee displays greater flexion angles and angular velocities, joint power and work in response to the larger peak ground reaction force from 0.6-m height, compared to 0.3-m height. We further hypothesized that the knee exhibits elevated flexion angles and angular velocities, joint power and work during double-leg landing, relative to single-leg landing. Ground reaction force, knee joint kinematics and energetics data were obtained from 10 subjects performing single-leg and double-leg landing from 0.3-m to 0.6-m heights, using motion-capture system and force-plates. Higher peak ground reaction force (p<0.05) was observed during single-leg landing and/or at greater landing height. We found greater knee flexion angles and angular velocities (p<0.05) during double-leg landing and/or at greater landing height. Elevated knee joint power and work were noted (p<0.05) during double-leg landing and/or at greater landing height. The knee joint is able to respond more effectively in terms of kinematics and energetics to a larger landing impact from an elevated height during double-leg landing, compared to single-leg landing. This allows better shock absorption and thus minimizes the risk of sustaining lower extremity injuries.

Limited hip and knee flexion during landing is associated with increased frontal plane knee motion and moments

BACKGROUND

It has been proposed that female athletes who limit knee and hip flexion during athletic tasks rely more on the passive restraints in the frontal plane to deceleration their body center of mass. This biomechanical pattern is thought to increase the risk for anterior cruciate ligament injury. To date, the relationship between sagittal plane kinematics and frontal plane knee motion and moments has not been explored.

METHODS

Subjects consisted of 58 female club soccer players (age range: 11-20 years) with no history of knee injury. Kinematics, ground reaction forces, and surface electromyography were collected while each subject performed a drop landing task. Subjects were divided into two groups based on combined sagittal plane knee and hip flexion angles during the deceleration phase of landing (high flexion and low flexion).

FINDINGS

Subjects in the low flexion group demonstrated increased knee valgus angles (P=0.02, effect size 0.27), increased knee adductor moments (P=0.03, effect size 0.24), decreased energy absorption at the knee and hip (P=0.02, effect size 0.25; and P<0.001, effect size 0.59), and increased vastus lateralis EMG when compared to subjects in the high flexion group (P=0.005, effect size 0.35).

INTERPRETATION

Female athletes with limited sagittal plane motion during landing exhibit a biomechanical profile that may put these individuals at greater risk for anterior cruciate ligament injury.

Differences in kinematics of single leg squatting between anterior cruciate ligament-injured patients and healthy controls

Seventy to eighty percent of all anterior cruciate ligament (ACL) injuries are due to non-contact injury mechanisms. It has been reported that the majority of injuries due to single leg landing come from valgus positioning of the lower leg. Preventing valgus positioning during single leg landing is expected to help reduce the number of ACL injuries. We found that many ACL-deficient patients cannot perform stable single leg squatting. Therefore, we performed 3D motion analysis of the single-legged half squat for ACL-injured patients to evaluate its significance as a risk factor for ACL injuries. We evaluated the relative angles between the body, thigh, and lower leg using an electromagnetic device during single leg half squatting performed by 63 ACL-injured patients (32 males, 31 females) the day before ACL reconstruction and by 26 healthy control subjects with no knee problems. The uninjured leg of ACL-injured male subjects demonstrated significantly less external knee rotation than that of the dominant leg of the male control. The uninjured leg of ACL-injured female subjects demonstrated significantly more external hip rotation and knee flexion and less hip flexion than that of the dominant leg of the female control. Comparing injured and uninjured legs, the injured leg of male subjects demonstrated significantly less external knee and hip rotation, less knee flexion, and more knee varus than that of the uninjured leg of male subjects. The injured leg of female subjects demonstrated more knee varus than that of the uninjured leg of female subjects. Regarding gender differences, female subjects demonstrated significantly more external hip rotation and knee valgus than male subjects did in both the injured and uninjured legs (P < 0.05). The current kinematic study exhibited biomechanical characteristics of female ACL-injured subjects compared with that of control groups. Kinematic correction during single leg half squat would reduce ACL reinjury in female ACL-injured subjects.

Effect of axial load on anterior tibial translation when transitioning from non-weight bearing to weight bearing

BACKGROUND

While the application of compressive joint loads and thigh muscle activity are associated with anterior tibial translation in vitro, less is known during early load acceptance in vivo. We investigated the effects of increasing axial loads on anterior tibial translation and thigh muscle activity in healthy knees during transition from non-weight bearing to early weight bearing.

METHODS

Participants (11 males, 11 females) underwent 20%, 40%, and 60% body weight acceptance trials at 20 degrees knee flexion while electromagnetic sensors measured anterior tibial translation (mm), and surface electromyography recorded quadriceps and hamstring muscle onset times (ms) and amplitudes (% maximal voluntary isometric contraction). Repeated measures ANOVA compared values across loads. Pearson correlations examined relationships between anterior tibial translation and muscle onset times and amplitudes within each load.

FINDINGS

As load increased, anterior tibial translation (Mean (standard deviation)) (20%=4.7 (1.7) mm<40%=7.1 (1.9) mm<60%=8.8 (2.1) mm), and quadriceps (20%=23.6 (14.9)% maximal voluntary isometric contraction <40%=32.7 (11.8)% maximal voluntary isometric contraction <60%=41.1 (13.5)% maximal voluntary isometric contraction) and hamstring (20%=15.5 (15.7)% maximal voluntary isometric contraction <40%=23.0 (16.4)% maximal voluntary isometric contraction <60%=27.6 (19.1)% maximal voluntary isometric contraction) activation increased, while quadriceps (20%=96.7 (28.4) ms>60% 80.2 (21.8) ms) and hamstring (20%=141.5 (65.0) ms and 40%=126.3 (68.8)>60% 107.6 (28.4) ms) onset times decreased (P0.05). There were no relationships between anterior tibial translation and muscle activation amplitudes (R=0.033-0.294) or onset times (R=-0.031-0.374) (P>0.09).

INTERPRETATION

Greater axial loads near full knee extension during early weight acceptance result in greater anterior tibial translation, regardless of faster and stronger activation amplitudes. These findings support injury prevention programs aimed to reduce impact forces as they may in turn reduce anterior tibial translation and corresponding ligamentous strain during dynamic activity.

A prospective investigation of biomechanical risk factors for patellofemoral pain syndrome: the Joint Undertaking to Monitor and Prevent ACL Injury (JUMP-ACL) cohort

BACKGROUND

Patellofemoral pain syndrome is one of the most common chronic knee injuries; however, little research has been done to determine the risk factors for this injury.

HYPOTHESIS

Altered lower extremity kinematics and kinetics, decreased strength, and altered postural measurements will be risk factors.

STUDY DESIGN

Cohort study (prognosis); Level of evidence, 2.

METHODS

A total of 1597 participants were enrolled in this investigation and prospectively followed from the date of their enrollment (July 2005, July 2006, or July 2007) through January 2008, a maximum of 2.5 years of follow-up. Each participant underwent baseline data collection during their pre-freshman summer at the United States Naval Academy. Baseline data collection included 3-dimensional motion analysis during a jump-landing task, 6 lower extremity isometric strength tests, and postural alignment measurements (navicular drop and Q angle).

RESULTS

Risk factors for the development of patellofemoral pain syndrome included decreased knee flexion angle, decreased vertical ground-reaction force, and increased hip internal rotation angle during the jump-landing task. Additionally, decreased quadriceps and hamstring strength, increased hip external rotator strength, and increased navicular drop were risk factors for the development of patellofemoral pain syndrome.

CONCLUSION

Multiple modifiable risk factors for patellofemoral pain syndrome pain have been identified in this investigation. To decrease the incidence of this chronic injury, the risk factors for patellofemoral pain syndrome need to be targeted in injury prevention programs.

CLINICAL RELEVANCE

Prevention programs should focus on increasing strength of the lower extremity musculature along with instructing proper mechanics during dynamic movements to decrease the incidence of patellofemoral pain syndrome.

Variability of motion in individuals with mechanical or functional ankle instability during a stop jump maneuver

BACKGROUND

Movement variability may influence episodes of instability following lateral ankle sprain.

METHODS

Sixty-three recreational athletes with a history of moderate-severe ankle sprain were recruited. Mechanically and functionally unstable ankle groups had 2 episodes of instability in the last year. Mechanically unstable had clinically lax lateral ankle ligaments; functionally unstable and copers did not. Copers had a history of sprain but no residual instability. Lower extremity 3-dimensional kinematics and ground reaction forces were measured during a 2-legged stop jump. Average ensemble curves of eight trials normalized to 100% of stance phase were created. The coefficient of variation and average standard deviation of the ensemble curves of each variable were identified. A log(e) (ln) transformation was performed on the data. One-way ANOVAs with Tukey post hoc testing were utilized with alpha=0.05.

FINDINGS

The functionally unstable group demonstrated greater mean (standard deviation) ln coefficient of variation ankle inversion/eversion 3.56 (1.19) than the mechanically unstable 2.77 (0.95) and copers 2.74 (1.05) (P=0.05 and P=0.04; eta(p)(2)=0.12), and greater ln standard deviation ankle inversion/eversion 1.07 (0.78) than copers 0.61 (0.31) (eta(p)(2)=0.13). The mechanically unstable group demonstrated greater ln coefficient of variation anterior-posterior ground reaction force 3.69 (0.27) than functionally unstable 3.43 (0.25) (P=0.02; eta(p)(2)=0.13).

INTERPRETATION

Functionally unstable individuals demonstrated greater ankle frontal plane movement variability during a stop jump, which may increase risk of instability. Mechanically unstable participants demonstrated greater anterior-posterior ground reaction force variability, which may indicate difficulty mitigating landing forces with lax ligaments. Movement variability may influence episodes of ankle instability.

Thigh strength and activation as predictors of knee biomechanics during a drop jump task

PURPOSE

To examine whether normalized quadriceps and hamstring strength would predict quadriceps and hamstring muscle activation amplitudes and whether these neuromuscular factors would predict knee kinematics and kinetics during a drop jump task.

METHODS

Thirty-nine females and 39 males were measured for isometric quadriceps and hamstring strength and were instrumented to obtain surface electromyography, kinematic, and kinetic measures during the initial landing of a drop jump. Multiple linear regressions first examined the relationship between thigh strength and activation then examined whether these neuromuscular variables were predictive of hip and knee flexion excursions, knee extensor moments (KEM), and anterior knee shear forces during the deceleration phase of the drop jump.

RESULTS

Females versus males produced lower normalized thigh strength and demonstrated greater quadriceps and hamstring activation amplitudes during the drop jump. Lower thigh muscle strength was a weak (males) to moderate (females) predictor of greater quadriceps activation amplitudes. However, thigh strength and activation were poor predictors of hip and knee joint excursions and KEM. Regardless of sex and thigh strength, anterior shear forces were greater in individuals who demonstrated less hip flexion and greater knee flexion excursions and greater peak quadriceps activation and internal KEM during the landing.

CONCLUSIONS

Although thigh muscle strength explained some of the variance in quadriceps and hamstring activation levels as measured with surface electromyography, we failed to support the hypothesis that these neuromuscular factors are strong predictors of sagittal plane hip and knee flexion excursions or KEM. Although greater quadriceps activation amplitude was a significant predictor of greater anterior tibial shear forces, its contribution was relatively small compared with kinematic and kinetic variables.