Stimulation site and phase modulation of the withdrawal reflex during gait initiation

OBJECTIVE: To investigate how the nociceptive withdrawal reflex (NWR) is modulated during gait initiation. METHODS: The NWR was elicited in ten subjects using electrical stimulation at four sites in the right foot during symmetrical stance (50% of body weight on each foot) or while performing the first step during gait initiation: either during heel off (HO, 20% of body load on the starting leg) or heel contact (HC, 80% of body load on the starting leg in the first step). Kinematics and EMG responses from major muscles of the ipsilateral leg were recorded. RESULTS: The NWR was modulated by stimulation site in all muscles except Soleus. The NWR responses elicited after stimulation of the arch were significantly larger than those evoked at all other sites in Tibialis Anterior, Rectus Femoris, and Vastus Lateralis. At the hip joint, the largest flexion was obtained during HO, whereas the smallest flexion was observed during HC, both following stimulation on the arch of the foot. CONCLUSIONS: The NWR responses were modulated to maintain balance and continue the development of the gait initiation process. SIGNIFICANCE: The NWR modulation followed a functional principle, which might allow a functional use in rehabilitation strategies.