Shoulder reflexes integrate elbow information at "long-latency" delay throughout a corrective action.

Previous studies have demonstrated a progression of function when healthy subjects counter a sudden mechanical load. Short-latency reflexes are linked to local stretch of the particular muscle and its antagonist. Long-latency reflexes integrate stretch information from both local sources and muscles crossing remote joints appropriate for limb's mechanical interactions. Unresolved is how sensory information is processed throughout the corrective response since capabilities at some time can be produced by circuits acting at that delay and at a briefer delays. One possibility is that local abilities are always expressed at a short-latency delay and integrative abilities are always expressed at a long-latency delay. Alternatively, the neural circuits may be altered over time leading to a temporal shift in expressing certain abilities; a refractory period could retard integrative responses to a second perturbation whereas priming could enable integrative responses at short-latency. We tested between these three hypotheses in a shoulder muscle by intermixing trials of step torque with either torque pulses (Experiment 1) or double-steps of torque (Experiment 2). The second perturbation occurred at 35ms, 60ms, and 110ms after the first perturbation to probe processing throughout the corrective action. The second perturbation reliably evoked short-latency responses in the shoulder muscle linked to only shoulder motion and long-latency responses linked to both shoulder and elbow motion. This pattern is best accounted by the continuous action of controllers with different delays and fixed processing abilities.