Sex comparison of extensibility, passive, and active stiffness of the knee flexors

Objective. To compare extensibility, and passive and active stiffness of the knee flexors between males and females. Design. An experimental design utilized 15 males and 15 females to identify sex differences in active extensibility, and active and passive stiffness of the knee flexors. Background. Muscle stiffness appears to contribute to joint stability from both mechanical and neuromuscular perspectives. Differences in knee flexor stiffness may partially explain higher female anterior cruciate ligament injury rates. Methods. Active knee flexor extensibility was assessed as subjects extend the knee from a fixed hip position, measuring the final knee position. Passive knee flexor stiffness was calculated as the slope of the moment-angle curve resulting from controlled passive knee extension. Active knee flexor stiffness was assessed by loading the lower extremity with 10% total body mass, and measuring the damping effect of the knee flexors on imposed vibratory motion about the knee joint. Results. Females displayed greater active extensibility (P<0.05), while males displayed greater active (P<0.05) and passive (P<0.05) knee flexor stiffness. Sex differences in active and passive knee flexor stiffness were not significant following normalization to anthropometric characteristics. Conclusions. The knee flexor musculature in males is less extensible and displays greater active and passive stiffness compared to females. However, these differences may be functions of greater mass and height in males.