Evoked human cutaneous reflexes during standing and step initiation.

Abstract

Cutaneous reflexes have been shown to be task dependent, phase dependent and stimulus intensity dependent in movement tasks (e.g. walking and obstacle avoidance). We have demonstrated previously that cutaneous input can reduce reaction times and alter anticipatory postural adjustments that precede a step. It is not known how cutaneous input produces these changes. The influence of cutaneous reflexes during step initiation and standing was assessed in young, healthy subjects (n=15). Two sets of experiments were conducted. In the first experiment subjects stood on a force platform, then initiated three steps as fast as possible, to either a visual or sural go cue. For each "go" cue, a sural stimulation (2 Radiating Threshold) was delivered at two phases, loading and unloading phase of step initiation. Fifteen trials were acquired for each go cue during each of the two phases. Average reflex responses were determined from tibialis anterior (TA) muscle. This task was again repeated after the second experiment. In the second experiment subjects stood for 40 sec on a wooden platform and reflexes were evoked for varying stimulus intensities. Average evoked responses from TA were obtained. During step initiation, the primary effect in TA was a long latency excitation (70 - 90 ms). The results obtained demonstrate (1) Effect of cueing- reflexes were modulated during visual cueing vs. reversed signs during sural cueing. (2) Effect of phase of step initiation- greater amplitude for the loading than the unloading phase for both go cues. (3) Effect of task- net reflex response was primarily excitatory during step initiation but primarily inhibitory during standing. The cutaneous reflex responses for step initiation suggest behaviorally appropriate modulation of the reflexes which may play a role in the earlier release of the step and enhancement of APAs that we have previously reported (Kukulka et al, 2009).