The influence of perturbation duration and velocity on the long-latency response to stretch in the biceps muscle.

Different neural pathways are proposed to mediate the long-latency stretch reflex response (M2) in muscles spanning distal and proximal joints of the upper limb. The M2 at the wrist joint is present only if the duration of the perturbation exceeds a critical time. Lee and Tatton put forward a converging input hypothesis, requiring an interaction of excitatory volleys at the spinal cord, to account for this feature. The goal of the present study was to examine the influence of the duration of perturbation on M2 responses elicited in a muscle spanning the elbow joint. Reflex responses were induced in the biceps brachii muscle by applying ramp-and-hold position displacements to the elbow. It was found that the M2 was strongly dependent on the duration of the perturbation. On average, responses were not elicited following perturbations of less than 35+/-5 ms. Using a novel double-movement paradigm, we were unable to provide support for the converging input hypothesis. The effect of the duration of perturbation on the M2 may account for the conflicting characteristics of the M2 that have been provided by previous studies applying mechanical or electrical perturbations of varying time durations.