Muscular responses and movement strategies during stumbling over obstacles

Muscular responses and movement strategies during stumbling over obstacles. J. Neurophysiol. 83: 2093-2102, 2000. Although many studies have investiga ted reflexes after stimulation of either cutaneous or proprioceptive affere nts, much less is known about responses after more natural perturbations, s uch as stumbling over an obstacle, in particular, the phase dependency of t hese responses and their relation to the stumbling behavior has received li ttle attention. Hence response strategies during stumbling reactions after perturbations at different times in the swing phase of gait were studied. W hile subjects walked on a treadmill, a rigid obstacle unexpectedly obstruct ed the forward sway of the foot. All subjects showed an "elevating strategy " after early swing perturbations and a "lowering strategy" after late swin g perturbations. During the elevating strategy, the foot was directly lifte d over the obstacle through extra knee flexion assisted by ipsilateral bice ps femoris (iBF) responses and ankle dorsiflexion assisted by tibialis ante rior (iTA) responses. Later, large rectus femoris (iRF) activations induced knee extension to place the foot on the treadmill. During the lowering str ategy, the foot was quickly placed on the treadmill and was lifted over the obstacle in the subsequent swing. Foot placement was actively controlled b y iRF and iBF responses related to knee extension and deceleration of the f orward sway. Activations of iTA mostly preceded the main ipsilateral soleus (iSO) responses. For both strategies, four response peaks could be disting uished with latencies of similar to 40 ms (RP1), similar to 75 ms (RP2), si milar to 110 ms (RP3), and similar to 160 ms (RP4). The amplitudes of these response peaks depended on the phase in the step cycle. The phase-dependen t modulation of the responses could not be accounted for by differences in stimulation or in background activity and therefore is assumed to be premot oneuronal in origin. In mid swing. both the elevating and lowering strategy could occur. For this phase, the responses of the two strategies could be compared in the absence of phase-dependent response modulation. Both strate gies had the same initial electromyographic responses till similar to 100 m s (RP1-RP2) after perturbation. The earliest response (RP1) is assumed to b e a short-latency stretch reflex evoked by the considerable impact of the c ollision, whereas the second (RP2) has features reminiscent of cutaneous an d proprioceptive responses. Both these responses did not determine the beha vioral response strategy. The functionally important response strategies de pended on later responses (RP3-RP3). These data suggest that during stumbli ng reactions, as a first line of defense, the CNS releases a relatively asp ecific response, which is Followed by an appropriate behavioral response to avoid the obstacle.