Evidence of phase-dependent nociceptive reflexes during locomotion in man

In 10 healthy subjects freely walking along a straightline, the effects of painful sural nerve stimulation, applied in different phases of the step cycle, were investigated on two antagonistic muscles of the ipsilateral lower limb acting on the knee joint: vastus lateralis (VL) and biceps femoris caput breve (BF). A clear-cut modulation in the amplitude (area) of the net reflex responses was consistently observed in both the motor nuclei explored. The extensor muscle, VL, exhibited a long-latency (mean 122 ms) reflex response, which was maximally increased by stimuli applied toward the end of the swing and in the first half of the stance phase of the stride, whereas the response appeared to be gated during the transition from the foot-flat to forefoot-contact phase. A second facilitation period was brought about by stimuli delivered in the early swing. When the response occurred superimposed on the VL locomotor activity, suppression of the ongoing EMG preceded the reflex discharge. In the flexor, BF, the same stimulus elicited a short-latency (mean 57 ms) and a long-latency (mean 132 ms) reflex response. The former was maximal after stimulation around the toe-off phase and the latter was strikingly enhanced in the late swing, where it was preceded by suppression of the background locomotor EMG activity. Responses with intermediate features (latency 70 to 80 ms, duration 90 to 120 ms), probably resulting from the merging of the early and late components, might be evoked in addition, being greatest in the last swing and in the period preceding toe-off. The findings show that in man the reflex pattern evoked by a painful cutaneous stimulus during locomotion is determined by the phase of the step cycle during which the stimulus is delivered. A functional role in maintenance of postural balance during destabilizing withdrawal reactions is conceivable.