The effects of series elastic stiffness and cutaneous sensitivity on leg muscle reflex responses to unanticipated slips during walking.

Fall-related injuries are a large public health concern for older adults (OA). Delayed or inappropriate afferent signaling may hinder detection of balance perturbation (e.g., slip), predisposing OA to falls and less efficacious corrective actions. Perturbation detection is largely governed by proprioceptive acuity, consisting of sensory information from skin and musculotendinous receptors. While skin and muscle receptors' signaling thresholds increase with age, feedback from muscle may be uniquely delayed and diminished by age-related decreases in series elastic tissue stiffness which, during a rapid, unanticipated joint position change, could decrease the velocity of muscle stretch, delaying detection. Ultimately, the association between skin and muscle afferent signaling integrity and muscle reflex timing in response to walking balance perturbations has yet to be well established. This study evaluated the associations between plantar cutaneous sensitivity (PS) and Achilles tendon stiffness (kAT) to muscle reflex responsiveness of ankle plantar- and dorsiflexor muscles following treadmill-induced slip perturbations in younger and OA. Compared to younger adults (YA), we found that OA had lesser PS and kAT and delayed tibialis anterior excitation onset. However, neither sensory outcome associated with muscle onset times. Later in stance, OA exhibited earlier triceps surae onsets than YA. However, only in YA did earlier gastrocnemius activity associate with greater kAT, which may indicate an ankle dominant balance recovery strategy in YA. Our results point to specific age-related changes in the timing of neuromuscular corrections to mitigate instability, which may underlie age-related differences in balance recovery efficacy and subsequent injury severity.