A novel three-dimensional shoulder rhythm definition that includes overhead and axially rotated humeral postures

Although the orientations of the scapula and clavicle in space are difficult to measure experimentally, the existence of a mathematically quantifiable shoulder rhythm allows their estimation based on thoracohumeral orientation. This study quantified the shoulder rhythm for arm postures that represent the right-handed reachable volume. Fourteen male and fourteen female participants performed static arm postures spread over five arm elevation angles: 0°, 45°, 90°, 135° and 180°, three elevation planes: 0°, 45° and 90° and, three axial rotations: maximum internal, neutral, and maximum external rotation. Kinematic data was collected using a passive motion-tracking system. Bone rotations were calculated using Euler angles and continuous parsimonious prediction models were generated to describe the rhythm for each angle. Linear models were obtained for all scapular angles and for all clavicular angles except elevation, for which a quadratic model was derived. Axial rotation of the humerus did not influence scapular retraction/protraction and plane of elevation did not influence clavicular elevation (p>.05). Elevation angle was the largest contributor to lateral rotation and posterior tilt of the scapula and all clavicular angles, while plane of elevation was the largest contributor to scapular protraction. The shoulder rhythm models can be incorporated into existing and future shoulder biomechanical models to determine relative shoulder bony orientations and subsequently determine muscular capacities. The novelty of the described models is their deliberate consideration of axial humeral rotation and the inclusion of multiple overhead postures, the combination of which generates a rhythm based on an unprecedentedly comprehensive range of humeral postures.