Differences in the electromyographic activity of the hamstring muscles during maximal eccentric knee flexion.

This study investigated the effects of the knee joint angle and angular velocity on hamstring muscles' activation patterns during maximum eccentric knee flexion contractions. Ten healthy young males (23.4 +/- 1.3 years) performed eccentric knee flexion at constant velocities of 10, 60, 180, and 300 deg/s in random order. The eccentric knee flexion torque and the surface electromyographic (EMG) activity of the biceps femoris (BF), semitendinosus (ST), and semimembranosus (SM) muscles were measured. The results of torque during 10 deg/s were lower than the faster velocities. No significant change was found in eccentric torque output and the EMG amplitude with change in the faster test velocities, although those values showed a decreasing tendency as the knee approached extension. Furthermore, the EMG amplitude of the BF decreased significantly as the knee approached extension, although the EMG activity of the ST and SM remained constant. These results suggest that the neural inhibitory mechanism might be involved in decreasing in maximal voluntary force and hamstring muscles activation toward the knee extension during high-velocity eccentric movement and therefore subjects have difficulties to maintain high eccentric force level throughout the motion. Moreover, the possible mechanism reducing the BF muscle activation as the knee approaches extension was architectural differences in the hamstring muscles, which might reflect each muscle's function.