Phase-specific modulation of the soleus H-reflex as a function of threat to stability during walking.

The purpose of the present study was to determine whether the soleus H-reflex is modulated with changes in the level of postural threat during walking. H-reflexes were tested at four points in the step cycle when subjects walked in 5 conditions representing different levels of postural threat. H-reflexes were significantly increased in amplitude at heelstrike in conditions of increased postural threat compared to normal treadmill walking with only minimal changes in H-reflex amplitude at other step cycle points. Conversely when subjects walked while holding stable handles, to decrease postural threat, the amplitude of the H-reflex was significantly smaller at heelstrike and midstance compared to normal walking. The changes in the amplitude of the H-reflex between walking conditions were not accompanied by changes in ongoing electromyographic activity or movements. Our findings suggest that the amplitude of the reflex is adjusted in a phase-specific manner, related to the postural uncertainty of the task. These adaptations in reflex amplitude may be related to changes in the amplitude of corrective responses following perturbations during walking. The adaptations in the amplitude of the H-reflex specific to heelstrike may be important in the control of foot placement at ground contact.