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

The present study investigated whether short-latency stretch reflexes are present during human stumbling reactions. While subjects walked on a treadmill, the forward sway of the foot was unexpectedly obstructed with an obstacle. 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 obstructed 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 the corresponding muscles or variations in the perturbation. Hence, mechanisms at a premotoneuronal level might play a role in the active phase-dependent control of these responses. The coactivation of antagonists as well as the 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.