Radiographic evaluation of glenohumeral kinematics: A muscle fatigue model

The purpose of this study was to document the effect of muscle fatigue on g lenohumeral kinematics. Twelve male volunteers without shoulder disease and with an average age of 27 years were studied. Glenohumeral anteroposterior radiographs were taken at 45 degrees intervals as the arm was abducted in the plane of the scapula from 0 degrees to 135 degrees. This series of radi ographs was performed both before and immediately after the subject perform ed a series of deltoid and rotator cuff fatiguing exercises. The average hu meral head position or translation before and after muscle fatigue For each arm angle was compared. For all subjects, before Fatigue, the position of the humeral head was below the center of the glenoid for all angles of abdu ction. There was essentially no change in position of the humeral head in t he prefatigue state, as the arm was abducted from 0 degrees to 135 degrees with no more than an average 0.3 mm of total humeral head excursion. After Fatigue, excursion of the humeral head increased to an average of 2.5 mm be tween the tested positions. The position of the humeral head with the arm a t 0 degrees of abduction was lower or had migrated inferiorly compared with the rested state, with an average 1.2 mm significant increase in inferior translation. With the initiation of abduction, the humeral head demonstrate d significant superior migration or translation in all positions tested. Th is result has important implications for conservative treatment of shoulder impingement and underscores the importance of rehabilitation to maximize t he endurance and strength of the rotator cuff musculature.