Bilateral and unilateral shoulder girdle kinematics during humeral elevation.

BACKGROUND: Due to the complexity of the shoulder mechanism, shoulder complex models to describe the reachable workspace generally lack information on shoulder girdle kinematics.
METHODS: Shoulder girdle kinematic data were recorded using an optoelectronic tracking device. Five male and five female healthy subjects performed bilateral and unilateral humeral elevation in four planes anterior and posterior to the body.
RESULTS: Bilateral and unilateral shoulder girdle kinematics during humeral elevation were substantially different. Bilateral shoulder movements led to a smaller workspace and larger contractions-elongations. Similar patterns of shoulder girdle elevation and retraction accompanying humeral elevation appeared in all measured planes, known as the shoulder rhythm. The shoulder girdle changes its length as a quadratic function during humeral elevation. The shoulder girdle angular motion range is the largest in the non-elevated humerus position, it is reduced during humeral elevation and halves in humerus maximal elevated position.
CONCLUSIONS: To describe motions of the shoulder complex, a model is proposed composed of an inner joint representing shoulder girdle joints and an outer joint representing the glenohumeral joint. RELEVANCE: The proposed model offers the possibility to compare movement of shoulders with disorders to a normal pattern. It can also provide a basis for shoulder complex kinematic modeling, biomechanical analysis, motion characteristics and exact calculation of the humerus reachable workspace.