Synergy of medial and lateral hamstrings at three positions of tibial rotation during maximum isometric knee flexion.

Rotation of the knee has been used to isolate the strength of the medial and lateral hamstrings during manual testing of the knee flexors. The purpose of this study was to determine if medial and lateral rotation of the knee during manual knee flexor strength testing increased the electromyographic activity of the respective hamstrings. Twenty-three women between 22 and 36 years old with no history of lower extremity injury or disease participated in the study. Indwelling fine wire electrodes were used to record EMG activity of the medial (semitendinosus and semimembranosus) and lateral (long and short heads of the biceps femoris) hamstring muscles during maximally resisted knee flexion with neutral, medial, and lateral rotation of the knee. Repeated measures analysis of variance with post hoc Bonferroni adjustments were used to compare EMG activity across the three tests. EMG activity increased significantly for the target hamstrings during ipsilateral rotation (P<0.05). The semitendinosus had a mean activity of 109% Max. during medial rotation as opposed to 95% Max. during lateral rotation. The semimembranosus averaged 107 and 89% Max. in medial and lateral rotation respectively. Conversely, both the long and short head of the biceps muscle showed significantly higher activity (P<0.05) during lateral compared to medial rotation (110 and 108% compared to 93 and 97%, respectively). Even though the differences are statistically significant they ranged from 2 to 13% only of maximum activity, the clinical importance of this small change in EMG activity is questionable.