EMG-angle relationship of the hamstring muscles during maximum knee flexion

The aim of the present study was to investigate the EMG-joint angle relationship during voluntary contraction with maximum effort and the differences in activity among three hamstring muscles during knee flexion. Ten healthy subjects performed maximum voluntary isometric and isokinetic knee flexion. The isometric tests were performed for 5 s at knee angles of 60 and 90degrees. The isokinetic test, which consisted of knee flexion from 0 to 120degrees in the prone position, was performed at an angular velocity of 30degrees/s (0.523 rad/s). The knee flexion torque was measured using a KIN-COM isokinetic dynamometer. The individual EMG activity of the hamstrings, i.e. the semitendinosus, semimembranosus, long head of the biceps femoris and short head of the biceps femoris muscles, was detected using a bipolar fine wire electrode. With isometric testing, the knee flexion torque at 60degrees knee flexion was greater than that at 90degrees. The mean peak isokinetic torque occurred from 15 to 30degrees knee flexion angle and then the torque decreased as the knee angle increased (p<0.01). The EMG activity of the hamstring muscles varied with the change in knee flexion angle except for the short head of the biceps femoris muscle under isometric condition. With isometric contraction, the integrated EMGs of the semitendinosus and sernimembranosus muscles at a knee flexion angle of 60degrees were significantly lower than that at 90degrees. During maximum isokinetic contraction, the integrated EMGs of the semitendinosus, sernimembranosus and short head of the biceps femoris muscles increased significantly as the knee angle increased from 0 to 105degrees of knee flexion (p<0.05). On the other hand, the integrated EMG of the long head of the biceps femoris muscle at a knee angle of 60degrees was significantly greater than that at 90degrees knee flexion with isometric testing (p<0.01). During maximum isokinetic contraction, the integrated EMG was the greatest at a knee angle between 15 and 30degrees, and then significantly decreased as the knee angle increased from 30 to 120degrees (P<0.01). These results demonstrate that the EMG activity of hamstring muscles during maximum isometric and isokinetic knee flexion varies with change in muscle length or joint angle, and that the activity of the long head of the biceps femoris muscle differs considerably from the other three heads of hamstrings. (C) 2002 Published by Elsevier Science Ltd.