EMG activity of medial and lateral hamstrings at three positions of tibial rotation during low-force isometric knee flexion contractions.

The purpose of this investigation was to determine how the position of tibial rotation effects the EMG activity of the medial and lateral hamstrings during low-force isometric knee flexion contractions. Forty-five subjects (ages 18-35) with no history of lower extremity injury or disease volunteered for this study. While lying prone, and with surface EMG electrodes secured to the bellies of their right medial (semitendinous and semimembranosus) and lateral (long head of the biceps femoris) hamstring muscles, each subject held the knee in 45° of flexion for 8 s against 5% of their body weight. This was performed three times in each of the positions of neutral tibial rotation, external tibial rotation, and internal tibial rotation. The root-mean-square (RMS) of the EMG activity from these muscles was determined for each of the contractions. A repeated measures ANOVA was used to compare the RMS values of the two muscle groups in the three positions. The average RMS values (in microvolts [mV]) obtained were (means and standard deviation): medial hamstrings in external rotation: 50.74 ± 23.11; in neutral: 65.57 ± 25.35; in internal rotation: 70.73 ± 31.86; lateral hamstrings in external rotation: 66.08 ± 46.99; in neutral: 46.18 ± 39.34; in internal rotation: 27.68 ± 17.86. A statistically significant interaction was found between tibial rotation and hamstring muscle (p < 0.0001). These results are consistent with the presumed function of these muscles in that EMG activity in the medial hamstrings increased when the tibia was rotated internally, whereas the lateral hamstring EMG activity increased when the tibia was rotated externally.