Electromyographic changes in the gluteus medius during stair ascent and descent in subjects with anterior knee pain

Ascending and descending stairs is a provocative activity for anterior knee pain (AKP) patients. The gluteus medius (GM) acts on the lower extremity in the frontal plane and can affect forces at the knee. Determining activation patterns of the GM in patients with AKP can help identify efficacy of training the GM in this population. This study examined electromyographic (EMG) firing patterns in lower extremity muscles in subjects with AKP while ascending and descending stairs. Subjects in the AKP group (n=16) demonstrated general AKP for at least 2 months compared to the control group (n=12); neither group had any history of knee trauma. Subjects were instrumented with EMG electrodes on the vastus medialis oblique (VMO), vastus lateralis (VL), and GM. Retroreflective markers were placed on lower extremities to determine knee flexion angle, and frontal plane pelvis orientation at toe contact. Subjects then performed a series of five stair (height=18 cm) ascent and descent trials. Repeated measures analyses of variance were performed on EMG and kinematic variables, between the two groups and between the symptomatic and asymptomatic sides. In the AKP group the GM demonstrated delayed onset and shorter durations for stair ascent and shorter duration during descent. There were no significant differences between sides in the AKP group. Consistent with previous studies, subjects in the AKP group demonstrated no difference in the VMO onsets relative to VL onsets compared to the control group. Changes in neuromuscular activity patterns may be a result of a compensations strategy due to AKP. Training of GM and other hip muscles is warranted during rehabilitation of AKP patients.