Biomechanical basis of transfers for shoulder paralysis.

Elevation of the scapulohumeral joint can be executed by two different and distinct systems, both capable of separately performing this movement. The deltoid muscle is composed of several parts contracting independently, sometimes antagonistically between them and providing elevation in different directions. Parts two and three (Fick) of the deltoid are essential because they are lateral to the centers of rotation and work in the plane of the scapula. The supraspinatus and adjacent muscle system can produce compensatory movements with full elevation in the plane of the scapula. Lateral rotation is absolutely necessary for abduction in the coronal plane. Rotatory muscles inserted on the tuberosities of the proximal part of the humerus are more efficient than those inserted on the humeral diaphysis. Vertical and horizontal stabilization of the humeral head in the glenoid fossa is the result of a complex balance between several muscles and passive factors. Trapezius and serratus anterior are essential for the rotation of the scapula around its centers of rotation.