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

Falls in older adults often occur during unexpected gait termination, when rapid deceleration is required. Age-related declines in strength, proprioception, and coordination impair efficient distal-to-proximal braking, raising fall risk. Variable step lengths, ranging from "normal" steps to larger "big" steps used when hurrying, better reflect real-world walking demands and pose greater mechanical and neuromuscular challenges. This study investigated how age and step length affect muscle synergy complexity and joint coordination during unexpected gait stops.

Twelve healthy younger and twelve healthy older adults performed normal- and big-step stops in response to random visual cues during walking. Surface EMG from distal to proximal leg muscles, whole-body kinematics, and kinetic force data from the stopping leg during the 1-second period immediately following the gait stop were collected. Muscle synergies, negative work at the knee and hip, joint coordination (Full Width at Half Maximum [FWHM] and Standard Deviation of Continuous Relative Phase [SD CRP]) for ankle-knee, knee-hip, and ankle-hip pairs, and postural sway were analyzed. A two-way mixed ANOVA (Step × Age) evaluated main and interaction effects.

Big steps increased the number of synergies in both groups, with older adults requiring more synergies than younger adults. All participants shared a distal synergy (ankle muscles and knee extensor); however, older adults recruited additional proximal synergies (hip or back muscles) with overlapping temporal synergy activation. Younger adults generated more negative work at the knee, whereas older adults shifted effort proximally, increasing hip joint work. Older adults demonstrated longer FWHM and higher SD CRP across all joint pair. Postural sway was greater in older adults overall, especially during big steps, indicating an interaction effect.

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.