Proprioceptive perturbations of stability during gait.

Through recent studies, the role of proprioceptors in reactions to perturbations during gait has been finally somewhat better understood. The input from spindle afferents has been investigated with tendon taps, vibration and other forms of muscle stretches, including some resembling natural perturbations (stumbling, slips, and ankle inversions). It was found that activation of spindle afferents produces short-latency response (SLR), consistent with a fast spinal pathway. These reflexes induce relatively minor activation in the stretched muscles. A central question is whether stretch reflexes can occur for stimuli that are quite remote. Thus, a new study was made to examine whether foot sole vibration is able to elicit reflex responses in upper-leg muscles, for example by conduction of vibrations throughout the whole leg. SLR responses were indeed found not only in lower- but also in upper-leg muscles. Similarly during stumbling, SLR are observed throughout the whole limb, although the primary perturbation occurs at foot level. After the SLR, much stronger activations usually occur, with latencies (85 or 120ms) well below those seen in voluntary contractions. These late responses are much more selective and presumably linked to the maintenance of stability. The role of the I(b) afferents from the Golgi tendon organs (GTO) is less clear. From animal work, it is known that these afferents are very sensitive to active muscle contraction and that they play a role in providing reinforcing feedback to extensors during the stance phase. The available evidence supports this notion in humans but lack of selective activation methods precludes more conclusive confirmation.