81, 106 Proposed mechanisms include (1) anatomic reductions in the available space beneath the coracoacromial arch or within the supraspinatus outlet area leading to subacromial impingement, 129 (2) intrinsic tendon degeneration from eccentric overload, ischemia, aging, or inferior tissue properties, 106 and (3) scapular or humeral movement alterations compromising the rotator cuff tissues through subacromial or internal impingement. The pathogenesis of rotator cuff tendinopathy is not precisely known and is somewhat controversial however, a multifactorial etiology is likely. Repetitive impingement is one of multiple proposed mechanisms for the development of rotator cuff disease, as well as progression to partial or full-thickness rotator cuff tearing. Shoulder impingement has been defined as compression, entrapment, or mechanical irritation of the rotator cuff structures and/or long head of the biceps tendon either beneath the coracoacromial arch (subacromial) or between the undersurface of the rotator cuff and the glenoid or glenoid labrum (internal). The point prevalence of shoulder pain in certain sports or occupations can reach 40% or higher. With the exception of adhesive capsulitis, the majority of these shoulder complaints are related to occupational or athletic activities that involve frequent use of the arm at, or above, shoulder level. 12, 84, 117 The most frequently occurring problems include shoulder impingement and associated rotator cuff disease or tendinopathy which can progress to rotator cuff tears, 24, 84 as well as glenohumeral joint instability and adhesive capsulitis. Shoulder pain and associated glenohumeral joint movement dysfunctions are common and debilitating conditions. The available evidence in clinical trials supports the use of therapeutic exercise in rehabilitating these patients, while further gains in effectiveness should continue to be pursued. This suggests that attention to these factors is warranted in the clinical evaluation and treatment of these patients. Scapular kinematic alterations similar to those found in patient populations have been identified in subjects with a short rest length of the pectoralis minor, tight soft-tissue structures in the posterior shoulder region, excessive thoracic kyphosis, or with flexed thoracic postures. There is also evidence for altered muscle activation in these patient populations, particularly, reduced serratus anterior and increased upper trapezius activation. There is evidence of scapular kinematic alterations associated with shoulder impingement, rotator cuff tendinopathy, rotator cuff tears, glenohumeral instability, adhesive capsulitis, and stiff shoulders. The purpose of this manuscript is to (1) review the normal kinematics of the scapula and clavicle during arm elevation, (2) review the evidence for abnormal scapular and clavicular kinematics in glenohumeral joint pathologies, (3) review potential biomechanical implications and mechanisms of these kinematic alterations, and (4) relate these biomechanical factors to considerations in the patient management process for these disorders. There is a growing body of literature associating abnormal scapular positions and motions, and, to a lesser degree, clavicular kinematics with a variety of shoulder pathologies.
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