Widespread short-latency stretch reflexes and their modulation during stumbling over obstacles

The present study investigated whether short-latency stretch reflexes are p resent during human stumbling reactions, While subjects walked on a treadmi ll, the forward sway of the foot was unexpectedly obstructed with an obstac le. All subjects showed reflex responses with average latencies of 34-42 ms in both the upper and the lower leg flexors and extensors of the obstructe d leg. The amplitudes of these responses depended on the phase in the step cycle and were not strictly related to either the background activity of th e corresponding muscles or variations in the perturbation. Hence, mechanism s at a premotoneuronal level might play a role in the active phase-dependen t control of these responses. The coactivation of antagonists as well as th e lack of obvious kinesiologic consequences following the responses suggest that the short-latency responses may generate joint stiffness. This may be a first line of defense: in preparing for the functional reaction,which is generated by longer latency responses, in order to take appropriate action concerning the obstacle. (C) 1999 Elsevier Science B.V. All rights reserve d.