GENDER DIFFERENCES IN ACTIVE KNEE JOINT STIFFNESS Kevin P. Granata, PhD. and Sara E. Wilson, PhD Motion Analysis and Motor Performance Laboratory University of Virginia E-Mail : KPG8n @ Virginia.edu INTRODUCTION Active female suffer 2 to 8 times the number of musculoskeletal injuries as equivalently trained men for both knee injury and low- back pain 1,2 . Research suggests these injuries may be related to biomechanical instability. One of the primary contributing components to stability during functional tasks is the mechanical stiffness of active muscles 3 . Research demonstrated that active co-contraction increases total joint stiffness, less in women than men, suggesting gender differences active muscle stiffness 4 . Gender differences in joint stiffness from active muscle contraction may cause decreased stability in women and contribute to the greater risk of musculoskeletal injury. As part of a larger study of biomechanical stability we measured the active knee stiffness as a function of gender in a controlled (non weight bearing) experiment. METHODS Sixteen male and fourteen female healthy volunteers between the ages of 21 and 39 participated following informed consent. Subject’s thighs were securely fastened in to a isokinetic dynamometer with the lower leg free to move. Neutral ankle posture was maintained by means of a fixed ankle-foot orthosis (AFO). Subjects were required to support the lower leg and added weights of 0 kg, 6 kg and 20% max. at a knee flexion angle of 45 0 . A sudden transient perturbation, i.e. a quick tap thrusting the leg downward (Figure 1), was applied to the angle. The resulting knee flexion / extension motions were recorded by an accelerometer attached to the heel of the orthosis. Subjects were asked to maintain a Figure1. Second-order damped harmonic model (gray) was fit to the acceleration data (black). orthosis. Subjects were asked to maintain a constant muscle activity by monitoring an electromyographic display measured from bipolar surface electrodes over the belly of the biceps femoris (hamstring activity) and rectus femoris (quadriceps activity). Flexion / extension oscillations of the knee were used to calculate stiffness. Fast Fourier transforms were performed to obtain the damped natural frequency, and peak-to- peak analyses recorded the exponential decay of the oscillation. These data were applied to a second order model of knee motion to determine active muscle stiffness and damping. Multiple regression and ANOVA were performed to assess the influence of gender, knee moment, and muscle (hamstring vs quadriceps) on active joint stiffness. RESULTS The model accurately represented the measured motion data explained 69% of the data variability with RMS error less than 5.9 % of the baseline acceleration of gravity. There was no performance difference as a function of gender. The female subjects produced less than 57% of the active stiffness demonstrated by the -1.00 -0.90 -0.80 -0.70 -0.60 -0.50 0.00 1.00 2.00 3.00 time (s) Acclerations (g)