Increased human stretch reflex dynamic sensitivity with height‐induced postural threat

Key points Threats to standing balance (postural threat) are known to facilitate soleus tendon‐tap reflexes, yet the mechanisms driving reflex changes are unknown. Scaling of ramp‐and‐hold dorsiflexion stretch reflexes to stretch velocity and amplitude were examined as indirect measures of changes to muscle spindle dynamic and static function with height‐induced postural threat. Overall, stretch reflexes were larger with threat. Furthermore, the slope (gain) of the stretch‐velocity vs. short‐latency reflex amplitude relationship was increased with threat. These findings are interpreted as indirect evidence for increased muscle spindle dynamic sensitivity, independent of changes in background muscle activity levels, with a threat to standing balance. We argue that context‐dependent scaling of stretch reflexes forms part of a multisensory tuning process where acquisition and/or processing of balance‐relevant sensory information is continuously primed to facilitate feedback control of standing balance in challenging balance scenarios.