Dysfunction of hip stabilizing muscles such as quadratus femoris (QF) is identified as a potential source of lower extremity injury during functional tasks like running. Despite these assumptions, there are currently no electromyography (EMG) data that establish the burst activity profile of QF during any functional task like walking or running. The objectives of this study were to characterize and compare the EMG activity profile of QF while walking and running (primary aim) and describe the direction specific action of QF (secondary aim). A bipolar fine-wire intramuscular electrode was inserted via ultrasound guidance into the QF of 10 healthy participants (4 females). Ensemble curves were generated from four walking and running trials, and normalized to maximum voluntary isometric contractions (MVICs). Paired t-tests compared the temporal and amplitude EMG variables. The relative activity of QF in the MVICs was calculated. The QF displayed moderate to high amplitude activity in the stance phase of walking and very high activity during stance in running. During swing, there was minimal QF activity recorded during walking and high amplitudes were present while running (run vs walk effect size=4.23, P<0.001). For the MVICs, external rotation and clam produced the greatest QF activity, with the hip in the anatomical position. This study provides an understanding of the activity demands placed on QF while walking and running. The high activity in late swing during running may signify a synergistic role with other posterior thigh muscles to control deceleration of the limb in preparation for stance.