The axial rotation rhythm of upper extremity

To determine the dynamic axial rotation movement of human upper extremity, we developed a combined skin- and scapula-based marker system in which the scapula motion was detected with a marker set attached to an intracortical pin on the acromion. The subject performed axial rotation with the arm in 5 different positions. The total rotation range varied greatly if the arm was in different position. The pattern of axial rotation of each segment also changed. The upper extremity rotates total 376° at 90° abduction and only 257° at full abduction. The forearm usually contributed about 140°, so the differences mainly came from the scapulothoracic and the glenohumeral joints. The scapulothoracic joint axially rotated 66° with the arm in backward extension while only rotated 19° with the arm in 90° forward flexion. The glenohumeral joint rotated maximally, 143° with the arm at 90° abduction, while minimally, 69° with the arm at maximal abduction.