The gluteus medius and piriformis muscles have hip extension and
external rotation moment arms throughout the gait cycle [1,2].
Maximum activation peaks were found during the early swing phase as
well as early and late stance. During the latter peak muscle activation
event, the hip was internally rotating via a net hip internal rotation
torque. Thus gluteus medius and piriformis muscle activation was
increased because it was working antagonistically to control the rate or
extent of hip internal rotation.
The results indicate that the activation of the deep hip muscles
primarily functioned to control or stabilize transverse plane hip rotations
and contributed secondary functions to sagittal plane motion