The shoulder is a highly mobile joint spanned by complex musculature. Careful coordination of muscles is required to maintain shoulder integrity and stability. The objective of this study was to determine the role of stretch reflexes in maintaining this coordination. Specifically, we quantified whether stretch reflexes elicited in muscles spanning the glenohumeral joint were best described by homonymous pathways linking each muscle to itself, or heteronymous pathways coordinating activity between muscles. We used a novel robotic manipulandum to perturb shoulder posture in three degrees of freedom as subjects exerted isometric torques. Electromyograms were used to measure the elicited stretch reflexes. Our results suggest that stretch reflexes are coordinated between muscles, and that this coordination is mediated at least in part through rapid, spinal networks.