Loading characteristics of females exhibiting excessive valgus moments during cutting

Abstract

Background

Excessive knee valgus moments are considered a risk factor for non-contact anterior cruciate ligament injuries in female athletes, however, little is known about the biomechanical factors that contribute to this loading pattern. The purpose of this study was to compare lower extremity kinematics, foot position and ground reaction forces between female soccer players who demonstrate normal frontal plane moments at the knee with those who demonstrate excessive frontal plane moments at the knee during a cutting maneuver.

Methods

Sixty-one female soccer players, 16 (2) years, participated. Three dimensional kinematics and ground reaction forces were recorded during a side-step cutting maneuver. Knee frontal plane moments were calculated with inverse dynamics equations and were used to classify subjects into normal (N = 38) and excessive valgus moment (N = 23) groups.

Findings

Data revealed that the subjects with excessive valgus moments demonstrated an initial loading pattern that included greater laterally directed ground reaction forces (P < 0.001, effect size 1.51), increased hip abduction (P = 0.002, effect size 0.79), increased hip internal rotation (P = 0.008, effect size 0.71) and a more internally rotated foot progression angle (P = 0.04, effect size 0.55). Taken together, these variables explained 49% of the variance in peak knee valgus moment (R = .698, P < 0.001).

Interpretation

These results provide insight into potentially injurious loading strategies and support the premise that interventions designed to encourage loading of the lower extremity in a more neutral alignment may work to decrease frontal plane loading at the knee.

Introduction

In an attempt to explain the disproportionate incidence of anterior cruciate ligament (ACL) injury in female athletes, biomechanical studies have evaluated gender differences in knee joint mechanics during the performance of athletic tasks. It has been suggested that neuromuscular control strategies employed by females during these activities result in abnormal knee joint loading which place them at greater risk for ACL injury (Chappell et al., 2002, Hewett et al., 2006, McLean et al., 2004b). In particular, females have been found to perform cutting and landing tasks with less knee flexion (Malinzak et al., 2001) and greater knee valgus (Malinzak et al., 2001, McLean et al., 1999) when compared to males. Females also have been found to demonstrate greater external knee valgus moments than males when performing a stop jump task (Chappell et al., 2002) and a preplanned side-step cutting task (Sigward and Powers, 2006, McLean et al., 2005).

Excessive external knee valgus moments are thought to place females at greater risk for ACL injury. For example, a prospective study of 205 female athletes found that those who tore their ACL over the course of a season demonstrated knee valgus moments during a landing task that were 2.5 times greater than those who did not tear their ACL (Hewett et al., 2005). These findings are consistent with in vitro (Markolf et al., 1990) and modeling studies (Bendjaballah et al., 1997) that have shown increased load on the ACL in response to valgus torques applied to the knee.

While data implicating excessive knee valgus moments as a risk factor for ACL injury is building, little is known about the factors that contribute to this loading pattern at the knee. Although authors have suggested that altered hip kinematics (particularly adduction and internal rotation) may contribute to valgus loading of the knee (Ferber et al., 2003, Ireland, 2002, Leetun et al., 2004, Lephart et al., 2002, Pollard et al., 2007), only one study has evaluated the relationship between knee valgus moments and lower extremity kinematics. McLean et al. (2005) evaluated the association between knee valgus moments and lower extremity kinematics at initial contact in 20 male and female collegiate basketball players while performing a side step cutting maneuver. These authors reported that peak knee valgus moment was correlated with initial hip internal rotation angle (r = 0.76), initial knee valgus angle (r = 0.53) and initial hip flexion angle (r = 0.42). When stratified by gender, knee valgus moments of the female participants were more sensitive to changes in hip position than the male participants. While this study provides evidence that lower extremity kinematics are associated with frontal plane loading of the knee (particularly in females) further insight may be gained by expanding the analysis to include the magnitude and direction of the ground reaction forces. For example, a kinematic pattern that moves the knee joint center medial to the center of pressure (i.e. hip adduction, knee valgus); combined with larger ground reaction forces may result in increased valgus loading.

Evaluating the biomechanical contributors to excessive knee valgus loading may prove valuable in identifying individuals at risk for ACL injury. Furthermore, knowledge of potentially injurious lower extremity mechanics is needed for the development of effective ACL injury prevention programs. Therefore, the purpose of this study was to compare lower extremity kinematics, foot position and ground reaction forces between female soccer players who demonstrate normal frontal plane moments at the knee with those who demonstrate excessive frontal plane moments at the knee during a cutting maneuver. Based on previous work in this area, it was hypothesized that females with excessive frontal plane moments would demonstrate greater degrees of hip adduction and internal rotation, knee valgus and larger ground reaction forces.