The effects of muscle activity and fatigue on three-dimensional scapulothoracic and glenohumeral kinematics

D. David Ebaugh

doi:10.17918/00000691

Shoulder pain is common among individuals who regularly use their arms in an overhead repetitive manner. One plausible cause of shoulder pain is altered scapulothoracic and/or glenohumeral kinematics secondary to changes in muscle activity and/or fatigue. The purposes of this study were to investigate the effects of different levels of shoulder muscle activity and fatigue on scapulothoracic and glenohumeral kinematics in a healthy population. To examine the effects of different muscle activity levels on scapulothoracic kinematics, trials of active and passive arm elevation were performed. There was more upward rotation of the scapula, external rotation of the scapula, clavicular retraction, and clavicular elevation when the arm was raised actively. This was most pronounced for scapular upward rotation through the mid-range of arm elevation. To examine the effects of muscle fatigue on scapulothoracic and glenohumeral kinematics, two fatigue protocols (shoulder elevation and shoulder external rotation) were performed. Three-dimensional scapulothoracic and glenohumeral kinematics were collected prior to and immediately following the fatigue protocols. In the shoulder elevation fatigue protocol, subjects performed a series of tasks that required them to use their arms in a repetitive, overhead manner. Following the completion of the shoulder elevation fatigue protocol, subjects demonstrated more upward and external rotation of the scapula, more clavicular retraction, and less humeral external rotation during arm elevation. Following the completion of the external rotation fatigue protocol, subjects demonstrated more scapular upward rotation, more clavicular retraction, less posterior tilt of the scapula, and less external rotation of the humerus. Overall, this research demonstrates the important role that the shoulder girdle musculature has in the production and control of scapulothoracic motion. These studies have shown that altered levels of muscle activity and fatigue lead to alterations in scapulothoracic and glenohumeral kinematics. Additionally, the patterns of scapulothoracic and glenohumeral kinematic change were similar for the elevation and external rotation fatigue protocols. This suggests that there may be an inherent compensatory mechanism whereby scapulothoracic motion is altered in response to impairments of the rotator cuff muscles.

The effects of muscle activity and fatigue on three-dimensional scapulothoracic and glenohumeral kinematics

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The effects of muscle activity and fatigue on three-dimensional scapulothoracic and glenohumeral kinematics

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