Motor point delineation of the gluteus medius muscle for functional electrical stimulation: an in vivo anatomic study

The purpose of this study was to delineate the motor point region of the gluteus medius muscle to aid in placing epimysial electrodes for functional electrical stimulation. Seven surgically dissected gluteus medius muscles were studied in five patients. The lateral surface of the muscle was sequentially probed with an electrical stimulator. The motor point region, which responded maximally to an applied electric current, was located on the posterolateral aspect of the muscle, adjacent to the superior margin of the piriformis, 3 cm lateral to the greater sciatic notch; it was rectangular or oval, measuring approximately 3.5 cm by 3.0 cm. The mean threshold current that produced a contraction was 14 mA (range = 2 to 26 mA). The mean minimum current required to produce a maximum contraction was 34 mA (range = 11 to 60 mA). This information provides guidelines for the placement of electrodes for functional electrical stimulation of this deeply situated muscle, and it provides electrical stimulation parameters required for adequate muscle activation.