Abstract
The purpose of this study was to delineate differences between the medial and lateral hamstrings by comparing both electromyographic (EMG) activity and force output in three positions of tibial rotation. Forty-two normal subjects were positioned prone with their right leg secured in 90 deg of knee flexion. Surface electrodes were placed over the medial and lateral hamstring muscles. Each test session consisted of nine maximal isometric knee flexion contractions. Three contractions were recorded in each position of tibial rotation: neutral, external, and internal. Recordings of averaged EMG and force output were made by the Bio-Prompt 3000 and FARO Axis Muscle Tester, respectively. The results showed statistically significant differences: (1) lower force output when positioned in internal rotation (p < 0.01); (2) greater EMG activity of the medial hamstrings when positioned in neutral (p < 0.01); and (3) lower EMG activity of the lateral hamstrings when positioned in internal tibial rotation (p < 0.01). We concluded that these differences demonstrated the need to consider the hamstrings as separate muscles: medial and lateral.