Quantifying the Biomechanical and Neural Factors Contributing to Translational Shoulder StabilityPublic
The shoulder is the most mobile joint in the human body, allowing for the execution of complex and athletic tasks. Unfortunately, such expansive mobility comes at a cost. The shoulder is prone to instability, or painful symptoms associated with increased humeral head translation, and dislocation. To prevent dislocations and maintain shoulder stability while interacting with the environment, active contributions are required from shoulder muscles. The goal of active stability is to prevent excessive humeral head translations, yet how shoulder muscles actively resist these translations has not been quantified. In this thesis, I characterized how shoulder muscles increase glenohumeral stiffness—the resistance to humeral head translation that defines shoulder stability—and elucidated biomechanical and neural mechanisms that contribute to active stabilization. My results demonstrated that participants could nearly double their glenohumeral stiffness from passive levels while producing only 10% of their maximum shoulder torque, and the increase in glenohumeral stiffness was lower in a shoulder posture that is associated with symptoms of instability. I developed a computational model which demonstrated that shoulder muscles can increase glenohumeral stiffness through two mechanisms: contracting to compress the humeral head into the concave glenoid and resisting humeral head translations as they are stretched. My model suggested that muscles that primarily rotate the shoulder increase glenohumeral stiffness as much as rotator cuff muscles despite historical emphasis on the latter as the primary shoulder stabilizers. Finally, I explored the neurophysiology behind how activations are elicited in shoulder muscles due to humeral head translations. I found that stretch reflexes were elicited by humeral head translations in all shoulder muscles, and the magnitude of these responses were similar in rotator cuff muscles and primary shoulder movers. Together, my results characterized active stability in healthy shoulders and introduced experimental techniques to evaluate stabilizing deficits that may exist in populations with shoulder instability.
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