Coactivation of the knee extensors and flexors increases knee joint contact forces, which may lead to degradation of the articular surfaces. This study investigated the effect of neuromuscular fatigue induced by submaximal, repetitive, dynamic contractions on coactivation of knee musculature in young and middle-aged women. Data from 10 young women (24.6 ± 1.8 years) and 8 middle-aged women (55.4 ± 4.2 years) were analyzed. Measures included peak knee extension and flexion torques and the average amplitude of surface electromyography of rectus femoris and biceps femoris. Coactivation ratios were calculated from these activations. To induce fatigue, participants completed up to ten sets of 50 concentric knee extension and flexion contractions at 60°/s. A two-factor analysis of variance was used to determine the effect of age and fatigue. The young group showed higher peak torque compared with the middle-aged group (
P< .001). During flexion, biceps femoris activity increased after fatigue when both groups were considered together ( P= .018). During extension, biceps femoris activity was higher in the middle-aged than young group ( P= .043). Middle-aged women exhibited a trend for greater coactivation during knee extension compared with young women ( P= .066). This coactivation likely contributed to extension torque decrements in middle-aged women.