Despite extensive research, we still do not fully understand the biological mechanisms that underlie a female's increased susceptibility for suffering a noncontact ACL injury. While sex differences in neuromuscular control are often implicated, prevention efforts addressing these differences have not resulted in a profound or sustainable reduction in injury rates. This paper will explore two likely scenarios that explain this greater susceptibility in females: (1) females have a structurally weaker ligament that is more prone or susceptible to failure at a given load (scenario #1), or (2) females develop less knee protection and experiences higher relative loads on the ACL (scenario #2). While we have learned much over the last two decades about ACL injury risk in females, much remains unknown. Continued research is of paramount importance if we are to effectively identify those females who are at greatest risk for injury and effectively reduce their susceptibility through appropriate interventions.
ACL Injury Risk in the Physically Active: Why are Females More Susceptible?
Sandra J. Shultz
Recent Advances in Prevention of Primary and Secondary Anterior Cruciate Ligament Injury: What Does the Future Hold for Optimizing Knee-Joint Function?
Sandra J. Shultz and Randy J. Schmitz
Despite considerable advances in anterior cruciate ligament (ACL) injury-risk identification and prevention over the past 20 years, the annual incidence of ACL injury has continued to rise, and females remain at greater risk of both primary and secondary ACL injury. Important questions remain regarding ancillary risk factors we should target, the most effective training and rehabilitation approaches to ensure retention and transfer of learned skills from the rehabilitation setting to real-world sporting environment, and the development of more evidence-based criteria for return to sport that consider the whole athlete. As we look to the future, the optimization of primary and secondary ACL-injury prevention represents a complex, multidisciplinary problem with many unique and exciting opportunities to engage the various subdisciplines of kinesiology to address these emerging questions.
The Influence of Hip Structure on Functional Valgus Collapse During a Single-Leg Forward Landing in Females
Jennifer A. Hogg, Randy J. Schmitz, and Sandra J. Shultz
Clinical femoral anteversion (Craig test) and hip range of motion (ROM) have been associated with valgus collapse, but their clinical usefulness in predicting biomechanics is unknown. Our purpose was to determine the individual and combined predictive power of femoral anteversion and passive hip ROM on 3-dimensional valgus collapse (hip internal rotation and adduction, knee rotation, and abduction) during a single-leg forward landing in females. Femoral anteversion and passive hip ROM were measured on 20 females (24.9 [4.1] y, 168.7 [8.0] cm, 63.8 [11.6] kg). Three-dimensional kinematics and kinetics were collected over 5 trials of the task. Each variable was averaged across trials. Backward, stepwise regressions determined the extent to which our independent variables were associated with valgus collapse. The combination of greater hip internal and external rotation ROM (partial r = .52 and .56) predicted greater peak knee internal rotation moment (R 2 = .38, P = .02). Less hip internal rotation ROM (partial r = −.44) predicted greater peak knee abduction moments (R 2 = .20, P = .05). Greater total hip ROM (internal and external rotation ROM) was not consistently associated with combined motions of valgus collapse but was indicative of isolated knee moments. Passive hip ROM is more associated with knee moments than is femoral anteversion as measured with Craig test.
A Magnetic Resonance Imaging–Compatible Device to Perform In Vivo Anterior Knee Joint Loading
Kyoungyoun Park-Braswell, Sandra J. Shultz, and Randy J. Schmitz
Context: Greater anterior knee laxity (AKL) is associated with impaired sensory input and decreased functional knee stability. As functional magnetic resonance imaging (MRI) is the gold standard for understanding brain function, methods to load the anterior cruciate ligament in the MRI environment could further our understanding of the ligament’s sensory role in knee joint stability. Objective: To design and validate an MRI-compatible anterior knee joint loading device. Design: Descriptive laboratory study. Setting: University laboratory study. Participants: Sixteen healthy and physically active females participated (age = 23.4 [3.7] y; mass = 64.4 [8.4] kg). Interventions: The AKL was assessed by a commercially available arthrometer. The AKL was also assessed with a custom-made, MRI-compatible device that produced anterior knee joint loading in a manner similar to the commercial arthrometer while obtaining dynamic structural MRI data. Main Outcome Measurements: The AKL (in millimeters) at 133 N of loading was assessed with the commercial knee arthrometer. Anterior displacement of the tibia relative to the femur obtained at 133 N of loading was measured from dynamic MRI data obtained during usage of the custom device. Pearson correlations were used to examine relationships between the 2 measures. The 95% limits of agreement compared the absolute differences between the 2 devices. Results: There was a 3.2-mm systematic difference between AKL (6.3 [1.6] mm) and anterior tibial translation (3.2 [1.0] mm) measures. There was a significant positive correlation between values obtained from the commercial arthrometer and the MRI-compatible device values (r = .553, P = .026). Conclusions: While systematic differences were observed, the MRI-compatible anterior knee joint loading device anteriorly translated the tibia relative to the femur in a similar manner to a commercial arthrometer design to stress the anterior cruciate ligament. Such a device may be beneficial in future functional magnetic resonance imaging study of anterior cruciate ligament mechanoreception.
The Interday Measurement Consistency of and Relationships Between Hamstring and Leg Musculo-articular Stiffness
Justin P. Waxman, Randy J. Schmitz, and Sandra J. Shultz
Hamstring stiffness (KHAM) and leg stiffness (KLEG) are commonly examined relative to athletic performance and injury risk. Given these may be modifiable, it is important to understand day-to-day variations inherent in these measures before use in training studies. In addition, the extent to which KHAM and KLEG measure similar active stiffness characteristics has not been established. We investigated the interday measurement consistency of KHAM and KLEG, and examined the extent to which KLEG predicted KHAM in 6 males and 9 females. KHAM was moderately consistent day-to-day (ICC2,5 = .71; SEM = 76.3 N·m–1), and 95% limits of agreement (95% LOA) revealed a systematic bias with considerable absolute measurement error (95% LOA = 89.6 ± 224.8 N·m–1). Day-to-day differences in procedural factors explained 59.4% of the variance in day-to-day differences in KHAM. Bilateral and unilateral KLEG was more consistent (ICC2,3 range = .87–.94; SEM range = 1.0–2.91 kN·m–1) with lower absolute error (95% LOA bilateral= –2.0 ± 10.3; left leg = –0.36 ± 3.82; right leg = –1.05 ± 3.61 kN·m–1). KLEG explained 44% of the variance in KHAM (P < .01). Findings suggest that procedural factors must be carefully controlled to yield consistent and precise KHAM measures. The ease and consistency of KLEG, and moderate correlation with KHAM, may steer clinicians toward KLEG when measuring lower-extremity stiffness for screening studies and monitoring the effectiveness of training interventions over time.
The Role of Dynamic Hamstring Activation in Preventing Knee Ligament Injury
Sandra J. Shultz and David H. Perrin
Lower-Extremity Biomechanics and Maintenance of Vertical-Jump Height During Prolonged Intermittent Exercise
Randy J. Schmitz, John C. Cone, Timothy J. Copple, Robert A. Henson, and Sandra J. Shultz
Potential biomechanical compensations allowing for maintenance of maximal explosive performance during prolonged intermittent exercise, with respect to the corresponding rise in injury rates during the later stages of exercise or competition, are relatively unknown.
To identify lower-extremity countermovement-jump (CMJ) biomechanical factors using a principal-components approach and then examine how these factors changed during a 90-min intermittent-exercise protocol (IEP) while maintaining maximal jump height.
Fifty-nine intermittent-sport athletes (30 male, 29 female) participated in experimental and control conditions.
Before and after a dynamic warm-up and every 15 min during the 1st and 2nd halves of an individually prescribed 90-min IEP, participants were assessed on rating of perceived exertion, sprint/cut speed, and 3-dimensional CMJ biomechanics (experimental). On a separate day, the same measures were obtained every 15 min during 90 min of quiet rest (control).
Main Outcome Measures:
Univariate piecewise growth models analyzed progressive changes in CMJ performance and biomechanical factors extracted from a principal-components analysis of the individual biomechanical dependent variables.
While CMJ height was maintained during the 1st and 2nd halves, the body descended less and knee kinetic and energetic magnitudes decreased as the IEP progressed.
The results indicate that vertical-jump performance is maintained along with progressive biomechanical changes commonly associated with decreased performance. A better understanding of lower-extremity biomechanics during explosive actions in response to IEP allows us to further develop and individualize performance training programs.
The Effect of Menstrual-Cycle Phase on Hamstring Extensibility and Muscle Stiffness
David R. Bell, Megan P. Myrick, J. Troy Blackburn, Sandra J. Shultz, Kevin M. Guskiewicz, and Darin A. Padua
Preventing noncontact ACL injuries has been a major focus of athletic trainers and researchers. One factor that may influence female noncontact ACL injury is the fluctuating concentrations of hormones in the body.
To determine whether muscle properties change across the menstrual cycle.
Repeated measures. Testing was performed within 3 d after the onset of menses and ovulation. Repeated-measures ANOVAs were used to determine changes in variables across the menstrual cycle, and Pearson correlations were used to determine relationships between variables.
8 women with normal menstrual cycles.
Main Outcome Measures:
Active hamstring stiffness and hamstring extensibility.
Hamstring extensibility (P = .003) increased at the ovulation testing session but hamstring muscle stiffness (P = .66) did not.
The results indicate that hamstring muscle stiffness did not change across the menstrual cycle and hamstring extensibility increased at ovulation, when estrogen concentration increases.
Energy Absorption as a Predictor of Leg Impedance in Highly Trained Females
Anthony S. Kulas, Randy J. Schmitz, Sandra J. Shultz, Mary Allen Watson, and David H. Perrin
Although leg spring stiffness represents active muscular recruitment of the lower extremity during dynamic tasks such as hopping and running, the joint-specific characteristics comprising the damping portion of this measure, leg impedance, are uncertain. The purpose of this investigation was to assess the relationship between leg impedance and energy absorption at the ankle, knee, and hip during early (impact) and late (stabilization) phases of landing. Twenty highly trained female dancers (age = 20.3 ± 1.4 years, height = 163.7 ± 6.0 cm, mass = 62.1 ± 8.1 kg) were instrumented for biomechanical analysis. Subjects performed three sets of double-leg landings from under preferred, stiff, and soft landing conditions. A stepwise linear regression analysis revealed that ankle and knee energy absorption at impact, and knee and hip energy absorption during the stabilization phases of landing explained 75.5% of the variance in leg impedance. The primary predictor of leg impedance was knee energy absorption during the stabilization phase, independently accounting for 55% of the variance. Future validation studies applying this regression model to other groups of individuals are warranted.
The Effects of Gluteal Strength and Activation on the Relationship Between Femoral Alignment and Functional Valgus Collapse During a Single-Leg Landing
Jennifer A. Hogg, Terry Ackerman, Anh-Dung Nguyen, Scott E. Ross, Randy J. Schmitz, Jos Vanrenterghem, and Sandra J. Shultz
Context: A bias toward femoral internal rotation is a potential precursor to functional valgus collapse. The gluteal muscles may play a critical role in mitigating these effects. Objective: Determine the extent to which gluteal strength and activation mediate associations between femoral alignment measures and functional valgus collapse. Design: Cross-sectional. Setting: Research laboratory. Patients or Other Participants: Forty-five females (age = 20.1 [1.7] y; height = 165.2 [7.6] cm; weight = 68.6 [13.1] kg) and 45 males (age = 20.8 [2.0] y; height = 177.5 [8.7] cm; weight = 82.7 [16.5] kg), healthy for 6 months prior. Intervention(s): Femoral alignment was measured prone. Hip-extension and abduction strength were obtained using a handheld dynamometer. Three-dimensional biomechanics and surface electromyography were obtained during single-leg forward landings. Main Outcome Measures: Forward stepwise multiple linear regressions determined the influence of femoral alignment on functional valgus collapse and the mediating effects of gluteus maximus and medius strength and activation. Results: In females, less hip abduction strength predicted greater peak hip adduction angle (R 2 change = .10; P = .02), and greater hip-extensor activation predicted greater peak knee internal rotation angle (R 2 change = .14; P = .01). In males, lesser hip abduction strength predicted smaller peak knee abduction moment (R 2 change = .11; P = .03), and the combination of lesser hip abduction peak torque and lesser gluteus medius activation predicted greater hip internal rotation angle (R 2 change = .15; P = .04). No meaningful mediation effects were observed (υadj < .01). Conclusions: In females, after accounting for femoral alignment, less gluteal strength and higher muscle activation were marginally associated with valgus movement. In males, less gluteal strength was associated with a more varus posture. Gluteal strength did not mediate femoral alignment. Future research should determine the capability of females to use their strength efficiently.