Effect of muscle coactivation on spinal feedback processing

Determine whether and how simultaneous activation of agonist and antagonist muscles (muscle coactivation) modulates the processing of spinal proprioceptive feedback circuits, including short-latency stretch responses, during upper-limb tasks that engage both muscle groups, and specify the changes in excitability and response amplitude compared to reciprocal activation patterns induced by background loading.

Background

Decades of work have characterized spinal processing of proprioceptive feedback, especially short-latency stretch responses (SLRmechanical) and long-latency responses (LLRmechanical), typically under conditions using background loads that pre-excite agonists and inhibit antagonists. These paradigms produce reciprocal patterns of activation that differ fundamentally from conditions of voluntary muscle coactivation.

The paper argues that because coactivation requires active and reciprocal excitation of agonist and antagonist muscles, generalizations from load-based paradigms are limited, making it uncertain how even basic spinal feedback circuits behave under coactivation.