Tensor fascia latae and gluteal muscles myoelectric responses to increasing levels of hip medial rotation torque.

Medial hip rotation is typically attributed to the tensor fascia latae (TFL) and lateral rotation, to the gluteus maximus. However, experimental studies in cadavers suggest that the TFL lacks a moment arm for medial rotation and that the gluteus maximus may act as hip medial rotator depending on the hip flexion angle. In order to address this contradictory thinking, we measured the myoelectric activity of TFL, gluteus medius and gluteus maximus (superior portion, GMaxS, and inferior portion, GMaxI) for increasing levels of medial rotation torque applied to the hip. To keep frontal and sagittal plane hip joint net torques constant during the experiments, the medial hip rotation torque was changed by displacing standard weights along an aluminum bar device, thereby producing pure medial hip rotation torques. The effect of increasing medial hip rotation torque was investigated for a fully extended hip (0°), and at 45° and 90° of flexion. We found an increase in the myoelectric activity of the TFL (∼90%↑, p = 0.002) at 90° of flexion and of the GMaxS (∼7%↑, p = 0.048) at the extended position with an increase in medial hip torque application (from 0 to 7.4 N.m.). For the GMed (regardless of hip position) and for the 45° position (regardless of muscle), no systematic changes across torque conditions were observed. In contrast to the common clinical assumption and current practice thinking, our results indicate that an increase in TFL activity is required to control for an increase in external torque towards hip medial rotation.