Superior humeral head migration occurs after a protocol designed to fatigue the rotator cuff: A radiographic analysis

HYPOTHESIS: Awkward postures and repetitive work have been suggested to lead to shoulder fatigue, which may in turn decrease the subacromial space. The aim of this study was to quantify changes in humeral head position relative to the glenoid after rotator cuff fatigue. We hypothesized that the humeral head would migrate superiorly with fatigue due to the inability of the rotator cuff muscles to balance the upward pull of the deltoid. MATERIALS AND METHODS: Four anterior-posterior radiographs (at 0°, 45°, 90°, and 135° of elevation in the scapular plane) of the glenohumeral joint were taken before and after a fatiguing task. The fatiguing task was a simulated job task intended to exhaust the entire rotator cuff. RESULTS: The position of the humeral head with respect to the glenoid cavity was significantly affected both by arm elevation angle and fatigue state. In the prefatigued state, increasing arm elevation angle was related to superior translation until 90°, after which the humeral head moved inferiorly to a more central position. In the postfatigued state, the inability of the rotator cuff to centralize the humeral head led to increasing translations with higher elevations. DISCUSSION: Superior humeral head migration was associated with the fatigued state. This implies that overhead or repetitive work, or both, may accelerate the development of subacromial impingement through reduction of the subacromial space. CONCLUSIONS: Continuous overhead work demonstrably created rotator cuff fatigue, which apparently inhibited the ability of the shoulder musculature to resist upward humeral translation.