Modulation of lower limb withdrawal reflexes during gait: a topographical study.

The aim of this study was to investigate the modulation and topography of the nociceptive withdrawal reflex elicited by painful electrical stimulation of the foot sole during gait. Fifteen healthy volunteers participated in this study. Cutaneous electrical stimulation was delivered on five locations of the foot sole after heel-contact, during foot-flat, after heel-off, and during the mid-swing phase of the gait cycle during treadmill walking. Reflexes were recorded from muscles of the ipsilateral and contralateral legs. Furthermore, the kinematic responses in the sagittal plane of the ipsilateral ankle, knee, and hip joints were recorded. Reflexes in the distal muscles showed a site-dependent modulation. The largest responses in tibialis anterior were evoked at the arch of the foot and the smallest at the heel (P < 0.05). The largest soleus responses were also elicited at the arch of the foot (P < 0.04). The EMG responses in flexors and extensors of the knee and extensors of the contralateral leg were generally not dependent on the stimulation site. The response at the three joints showed site dependency, especially during the swing phase where maximal flexion was obtained by stimulation at the arch of the foot (P < 0.05). The withdrawal reflex was modulated during the gait cycle and presented distinctive characteristics for the different muscles studied. Minimal kinematic responses were observed during stance in contrast to swing phase. Modulation of the reflex probably ensures an appropriate withdrawal but primarily secures balance and continuity of movement.