Determination of functional rotation axes during elevation of the shouldercomplex

Study Design: A cross-sectional, descriptive study of shoulder movements co nducted on nonimpaired subjects. Objective: To investigate whether a single functional rotation axis about t he shoulder complex can be determined during elevation in the coronal or sa gittal planes, and to identify their location. Background: Accurate measurement of isokinetic torques about a joint requir es alignment of the dynamometer axis with an assumed rotation axis of the j oint. To assess shoulder function on a dynamometer, the location of a singl e rotation axis is not evident because the shoulder joint motion is based o n several anatomical joints. Therefore, the rotation axis where humerothora cic movements occur should be judged as a functional rotation axis. Methods and Measures: During slow elevation movements in the sagittal and c oronal plane, the position of the epicondyle and acromion were recorded wit h a motion analysis system. The motion trajectory of the elbow coordinates was fitted to a circle and considered an estimate of the functional shoulde r joint rotation axis in the specified plane. Results: The fitted trajectory appeared to be very accurate (root-mean-squa re error < 2%; N = 7). In the sagittal plane, the estimated functional rota tion axis was found at the humeral head; in the coronal plane, it was locat ed about 13 centimeters medial relative to the acromion. Conclusion: The shoulder complex of nonimpaired subjects can act as a hinge joint of the upper arm relative to the thorax during elevation in each mea sured plane.