Study Design - A cross-sectional,
descriptive study of shoulder movements conducted on nonimpaired subjects.
Objectives - To investigate whether a single functional rotation axis about
the shoulder complex can be determined during elevation in the coronal or sagittal
planes, and to identify their location.
Background - Accurate measurement of isokinetic torques about a joint requires
alignment of the dynamometer axis with an assumed rotation axis of the joint.
To assess shoulder function on a dynamometer, the location of a single rotation
axis is not evident because the shoulder joint motion is based on several anatomical
joints. Therefore, the rotation axis where humerothoracic movements occur should
be judged as a functional rotation axis.
Methods and Measures - During slow elevation movements in the sagittal and coronal
plane, the position of the epicondyle and acromion were recorded with a motion
analysis system (VICON). The motion trajectory of the elbow co-ordinates was
fitted to a circle and considered an estimate of the functional shoulder joint
rotation axis in the specified plane.
Results - The fitted trajectory appeared to be very accurate (root -mean-square
error < 2%; N = 7). In the sagittal plane, the estimated functional rotation
axis was found at the humeral head; in the coronal plane, it was located about
13 cm 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 measured
plane.