Purpose <p>Active stability is essential to preventing dislocations and the focus of rehabilitation following dislocations. This is thought to arise from shoulder muscles compressing the humeral head into the glenoid (called concavity compression). However, shoulder muscles may also resist humeral head translation through increases in intrinsic muscle stiffness, an unexplored mechanism. Our objective was to quantify shoulder muscles’ contributions to changes in glenohumeral stiffness, or the resistance to humeral head translation. We hypothesized that primary shoulder movers (e.g., the <i>pectoralis major</i> or <i>deltoid</i>) would differ from rotator cuff muscles in how much they increase glenohumeral stiffness because they leverage their intrinsic stiffness in addition to concavity compression.</p> Methods <p>We measured glenohumeral stiffness across a range of isometric muscle activation levels in shoulder abduction and used electromyography to estimate the contributions of rotator cuff muscles and primary shoulder movers. We then created a musculoskeletal model to evaluate individual muscle contributions to glenohumeral stiffness through both concavity compression and intrinsic muscle stiffness.</p> Results <p>We found that muscle activity in primary shoulder movers was a better predictor of active glenohumeral stiffness than in rotator cuff muscles (<i>R</i>² = 0.81 vs 0.36, <i>P</i> &lt; 0.001). Our musculoskeletal model demonstrated that concavity compression is the primary stabilizing mechanism for most shoulder muscles, yet the muscles that increase glenohumeral stiffness the most also do so considerably through their intrinsic muscle stiffness.</p> Conclusion <p>Our results emphasize the importance of primary shoulder movers as active stabilizers of the glenohumeral joint and highlight their potential importance in preventing shoulder dislocations.</p>