Age- and step-length-dependent alterations in muscle synergies and joint coordination during unexpected gait termination

Unexpected stopping highlights age-related changes in movement strategy. Older adults shift from a distal-driven braking to proximal, multi-joint co-contraction strategy, characterized by increased hip work, reduced knee work, and greater muscle synergy complexity. This proximal engagement reflects a tradeoff along the stability-efficiency continuum: it enhances safety but increases metabolic cost and postural sway. By disrupting the distal-to-proximal control sequence, this compensatory strategy likely increases postural sway and fall risk. Targeted rehabilitation focusing on eccentric quadriceps and ankle control, such as stand-to-sit and step-down drills, may help restore distal-driven balance recovery and reduce fall risk during adaptive (reactive) walking.