Glenohumeral muscle force and moment mechanics in a position of shoulder instability.

The three-dimensional orientation of the shoulder girdle musculature was studied in five cadaver shoulders in the position of function at 90 degrees of abduction and 90 degrees of external rotation using a method of computer assisted gross muscle cross-section analysis. The muscle volume, muscle fiber length, and physiological cross-sectional area were obtained by dissecting two specimens. The line of action, the magnitude and orientation of the moment were calculated for each muscle crossing the shoulder joint. The quantitative description of the moment potential of muscle forces influencing shoulder function was thus obtained. The most effective flexors of the shoulder which also appear to resist anterior dislocation in the position studied are the pectoral, the short head of the biceps, coracobrachialis, anterior deltoid, and the subscapularis. Most of the rotator cuff muscles and the posterior deltoid acted as adductors, while the anterior deltoid, long and short head of the biceps, and supraspinatus were abductors. In this position, external rotation was effected by the long head of the biceps, coracobrachialis, and the posterior deltoid, while the majority of the remaining muscles acted as internal rotators.