The effects of hip abductor fatigue on gait instability in older adults

Fatigue is a critical factor governing muscle force responsiveness and, thereby, capacity to respond to balance challenges. Mediolateral stability is crucial for older adults to safely navigate their daily environments and disproportionally requires active control; the hip abductors are the key muscles in regulating lateral foot placement and mediolateral stability. Our study aimed to characterize the effects of hip abductor fatigue on gait instability in a cohort of older adults. We hypothesized that fatigue, as induced by continuous lateral stepping, would compromise mediolateral walking balance control-evidenced by larger vulnerability to lateral waist-pull perturbations and decreased speed and accuracy when responding to prescribed changes in lateral foot placement. Here, we use the term vulnerability to signify some change in a specific balance outcome in response to a perturbation. Musculoskeletal modeling informed the magnitude of fatigue that compromises balance recovery and the ways in which other leg muscles compensate. Continuous lateral stepping induced significant hip abductor fatigue; on average, gluteus medius (GMed) electromyography (EMG) median frequency decreased by 16%, and, as measured after the balance challenge, maximum isometric force decreased by 9%. Fatigue significantly affected vulnerability to lateral waist-pull perturbations and elicited 14% larger lateral displacement, requiring 11% larger margins of stability during recovery. Conversely, performance on the lateral foot placement task was unaffected by fatigue. When modeling hip abductor fatigue more generally, our simulations of lateral balance recovery strides could accommodate, on average, up to 55% hip abductor fatigue. This study draws a mechanistic link between hip abductor fatigue and its compromising effect when called upon to preserve stability. These findings point to the importance of preventative exercises in reducing fatigability and preserving the ability to maintain balance.<b>NEW & NOTEWORTHY</b> Fatigue is a critical factor governing muscle force responsiveness and, thereby, capacity to respond to balance challenges. Mediolateral stability is crucial for older adults to safely navigate their daily environments and disproportionally requires active control; the hip abductors are the key muscles that regulate lateral foot placement and mediolateral stability. This research study draws a mechanistic link between hip abductor fatigue and its compromising effect when called upon to preserve lateral stability in older adults.