Individuals with diminished hip abductor muscle strength exhibit altered ankle biomechanics and neuromuscular activation during unipedal balance tasks.

Coordinated control of the hip and ankle is important for maintaining postural stability. The purpose of the study was to compare postural stability between individuals with contrasting hip abductor strength during unipedal balance tasks and to determine whether diminished hip abductor strength results in greater utilization of the ankle strategy to maintain balance. Forty-five females (276±35 years) participated in the study. Participants were ranked based on their isometric hip abductor muscle strength. The top 33% of the participants were categorized as the strong group (n=15) and the lower 33% as the weak group (n=15). Each subject performed a static and a dynamic unipedal balance task, during which mean COP displacement, peak ankle invertor and evertor moments, and neuromuscular activation of the lower leg muscles were assessed. Two-way mixed analyses of variance tests with task as a repeated factor were performed to detect the effects of task and group on the variables of interest. When averaged across tasks, mean medial-lateral COP displacement was significantly greater in the weak group (136±117 vs. 98±60 mm, p=0.05). The weak group also exhibited greater peak ankle invertor and evertor moments (0.31±0.10 vs. 0.25±0.11 Nm/kg, p=0.03; 0.04±0.06 vs. -0.02±0.07 Nm/kg, p=0.01), and increased peroneus longus activation (46±12 vs. 36±15%, p<0.01). Our results demonstrate that individuals with diminished hip abductor muscle strength demonstrated decreased medial-lateral postural stability, and exhibited a shift toward utilizing an ankle strategy to maintain balance during unipedal tasks.