Soleus and lateral gastrocnemius H‐reflexes during standing with unstable footwear

: Unstable footwear has been shown to increase lower extremity muscle activity, but the reflex response to perturbations induced by this intervention is unknown. Methods : Twenty healthy subjects stood in stable and unstable footwear conditions (presented randomly) while H‐reflex amplitude and background muscle activity were measured in the soleus and lateral gastrocnemius (LG) muscles. Results : Wearing unstable footwear resulted in larger H‐reflexes (normalized to the maximal M‐wave) for the LG (+12%; P  = 0.025), but not for the soleus (+4%; P  > 0.05). Background activity of both muscles was significantly higher in the unstable condition. Conclusions : The H‐reflex facilitation observed with unstable footwear was unexpected, as challenging postural conditions usually result in reflex depression. Increased muscle activity, decreased presynaptic inhibition, and/or more forward postural position may have (over‐)compensated the expected reflex depression. Differences between LG and soleus H‐reflex modulation may be due to diverging motor unit recruitment thresholds. Muscle Nerve 51 :764–766, 2015