Electromyography Activity of the Hamstring Muscles during Knee Flexion

Abstract

2340 Semitendinosus and gracilis tendons (ST/G) are commonly used as a replacement graft during anterior cruciate ligament (ACL) reconstruction. However, considerable weakness in the knee flexor muscles has been reported during deep knee flexion after ACL reconstruction using ST/G. The cause of this weakness remains controversial. PURPOSE: The purpose of this study was to investigate the relationship between knee flexion angle and electromyography (EMG) activity of the hamstring muscles. METHODS: Eleven volunteers (6 males, 5 females, age: 21 ± 3 years) participated in this study. Each subject in a prone position was asked to perform a maximum voluntary isometric contraction (MVC) for 5 seconds while unilateral knee flexion torque was measured using an isokinetic exercise machine (BIODEX) at 30, 45, 60, 90 and 105 degrees of knee flexion. EMG activity of the semimembranosus (SM), the semitendinosus (ST) and the biceps femoris (BF) during 50% of the MVC was detected using bipolar surface electrodes. EMG activity was also measured using bipolar fine wire electrodes to assess the reliability of the EMG activity as measured with the surface electrodes. The amplified and full-wave rectified EMG signals were integrated with respect to a 1-s period. RESULTS: Both the knee flexion torque and the integrated EMG (I-EMG) value of the SM was highest at 30 degrees of knee flexion and decreased as the knee flexion angle increased. The I-EMG value of the ST increased gradually as the knee flexion angle increased and was highest at 90 degree of knee flexion. The I-EMG value for the BF remained relatively constant with respect to knee flexion angle. EMG activities measured using fine wire electrodes were approximately the same as those values measured using surface electrodes. CONCLUSIONS: The results of this study denote that EMG activity of the hamstring muscles varies with respect to knee flexion angle. EMG activity of the ST during deep knee flexion was relatively higher than that of the SM and the BF, thus indicating that the removal of the ST tendon causes the decrease in deep knee flexion torque after ACL reconstruction using ST/G.