Shifts in the timing of gait muscle synergies in older adults with reduced lower-limb motor function.

Preventing frailty-related declines in lower-limb motor function is essential for extending healthy life expectancy. Early detection of motor function deterioration and a better understanding of muscle synergy characteristics can facilitate targeted interventions. Previous studies have linked alterations in synergy coordination patterns to severe motor deficits, but the characteristics associated with mild functional decline and the underlying transition processes remain unclear. Here, we analyzed gait-related muscle synergies in 14 community-dwelling older adults who were independent in daily life (7 females and 7 males; mean age = 72.0 ± 4.9 years). Lower-limb function was assessed using the Short Physical Performance Battery, and muscle synergies were extracted via non-negative matrix factorization, revealing four distinct synergies. Temporal characteristics (peak timing) were computed, and their correlation with lower-limb function was examined. Significant associations were observed that individuals whose synergy peak activations occurred closer in time (i.e., reduced temporal separation between synergy activations) exhibited poorer lower-limb function. In contrast, kinematic gait parameters such as hip, knee, and ankle joint angles showed no significant relationship with lower-limb function. These findings suggest that even in the absence of severe motor dysfunction, reduced temporal distinction between synergies may reflect an early alteration in neuromuscular coordination. Thus, temporal synergy parameters may serve as early biomarkers of frailty, providing new avenues for targeted interventions in aging populations.