12/23/2025
𝗠𝘂𝘀𝗰𝗹𝗲𝘀 𝗙𝘂𝗻𝗰𝘁𝗶𝗼𝗻𝗶𝗻𝗴 𝗮𝘀 𝗣𝗿𝗶𝗺𝗮𝗿𝘆 𝗦𝗵𝗼𝘂𝗹𝗱𝗲𝗿 𝗠𝗼𝘃𝗲𝗿𝘀 𝗔𝗶𝗱 𝘁𝗵𝗲 𝗥𝗼𝘁𝗮𝘁𝗼𝗿 𝗖𝘂𝗳𝗳 𝗠𝘂𝘀𝗰𝗹𝗲𝘀 𝗶𝗻 𝗜𝗻𝗰𝗿𝗲𝗮𝘀𝗶𝗻𝗴 𝗔𝗰𝘁𝗶𝘃𝗲 𝗚𝗹𝗲𝗻𝗼𝗵𝘂𝗺𝗲𝗿𝗮𝗹 𝗦𝘁𝗶𝗳𝗳𝗻𝗲𝘀𝘀
The glenohumeral joint relies heavily on active muscular control to maintain stability due to its limited bony constraint and high mobility. The rotator cuff muscles are widely considered the primary dynamic stabilizers, increasing resistance to humeral head translation through concavity compression of the humeral head into the glenoid (https://pubmed.ncbi.nlm.nih.gov/8504601/; https://pubmed.ncbi.nlm.nih.gov/15726085/). Consequently, rehabilitation strategies for shoulder instability emphasize early strengthening of the rotator cuff (https://pubmed.ncbi.nlm.nih.gov/22293772/).
💪 Muscles that primarily generate shoulder torque, such as the deltoid, latissimus dorsi/teres major and pectoralis major, have traditionally been viewed as potential destabilizers because of their anterior–posterior shear forces at the joint (https://pubmed.ncbi.nlm.nih.gov/19490400/). However, activation-dependent intrinsic muscle stiffness has been shown to enhance joint stability in other regions by directly resisting perturbations (Hogan, 1984; Franklin & Granata, 2007). The extent to which this mechanism contributes to translational stability of the shoulder remains unclear.
📘 A brand-new study by Nicolozakes and colleagues (https://pubmed.ncbi.nlm.nih.gov/40100611/) investigated how shoulder muscles contribute to active stabilization of the glenohumeral joint, with particular emphasis on translational joint stiffness, a key factor in preventing shoulder dislocation. Fifteen healthy adults performed submaximal isometric shoulder contractions while anterior–posterior perturbations were applied using a robotic system to quantify glenohumeral stiffness. Electromyography (EMG) was recorded from nine shoulder muscles, including both rotator cuff muscles and primary movers. Linear mixed-effects models were used to relate muscle activity to increases in active glenohumeral stiffness above passive levels. In parallel, a two-dimensional musculoskeletal model was developed to estimate individual muscle contributions to stiffness through concavity compression and intrinsic muscle stiffness mechanisms.
📊 Experimentally, active glenohumeral stiffness increased linearly with torque magnitude. Muscle activity in primary shoulder movers was a substantially better predictor of active glenohumeral stiffness than activity in rotator cuff muscles alone (R² = 0.81 vs. 0.36). Including all muscles provided the best predictive model, but rotator cuff activity explained only a modest additional proportion of variance. Collectively, primary shoulder movers increased active glenohumeral stiffness approximately three times more per unit activation than the rotator cuff.
📊 Musculoskeletal modeling supported these findings, demonstrating that although concavity compression is the dominant stabilizing mechanism for most muscles, intrinsic muscle stiffness makes a substantial contribution for muscles with larger anterior–posterior lines of action, particularly the pectoralis major, deltoid, and subscapularis. Sensitivity analyses showed that muscle line of action, fiber length, and glenoid curvature strongly influence the relative contributions of these mechanisms.
🏋️♀️ Conclusion for exercise-based rehabilitation
Overall, the study challenges the traditional view that rotator cuff muscles are the sole active stabilizers of the shoulder. The findings highlight the important stabilizing role of primary shoulder movers (deltoid, latissimus dorsi/teres major and pectoralis major), especially when the shoulder is abducted, and suggest that rehabilitation protocols for shoulder instability may benefit from earlier and more targeted strengthening of these muscles to enhance active glenohumeral stiffness and reduce dislocation risk.
📷 This figure illustrates a biomechanical model of glenohumeral joint stability in the axial plane. The humeral head articulates with the concave glenoid and can translate in the anterior–posterior direction.
Panel A shows that shoulder muscles differ in their lines of action. Muscles with a line of action aligned toward the glenoid primarily generate compressive forces, whereas muscles with more anterior or posterior orientations generate both compressive and shear components.
Panel B depicts intrinsic muscle stiffness, modeled as a spring-like property that increases with activation. When the humeral head is displaced, stretched muscles directly resist translation along their line of action.
Panel C shows the effect of glenoid curvature. As the humeral head translates, it also displaces laterally along the curved glenoid surface, creating a restoring force through concavity compression that resists further translation.
