Patients suffering tendon tears in the glenohumeral cuff muscles show activation of muscles which pull the arm downwards during arm elevation tasks. This so-called co-activation deviates from... Show morePatients suffering tendon tears in the glenohumeral cuff muscles show activation of muscles which pull the arm downwards during arm elevation tasks. This so-called co-activation deviates from healthy controls and is triggered by pain. Goal of this thesis was to demonstrate that deviating muscle activation in patients with glenohumeral cuff tears is related to shoulder instability. We hypothesized that they are compensatory for lost glenohumeral cuff functions, and restrict arm functionality. Cuff-tear simulations using a musculoskeletal model and an experimental arm loading protocol showed increased deltoid activation to compensate lost elevation moments, which jeopardizes glenohumeral stability. Activation of muscles pulling down the humeral head is required to compensate lost stability, which is counterproductive for arm elevation. There is a conflict between stability and mobility. This conflict can be solved by a tendon transfer of the teres major, which demonstrated shoulder function improvement and pain decrease. The concept of compensatory muscle activation provide insight in the underlying mechanisms of patients suffering glenohumeral cuff tears and potentially can be used, also at early symptoms like in impingement, as a diagnostic instrument or it can be applied in new treatment strategies. Show less
