Mechanics of Glenohumeral Instability.

Last updated Friday, February 04, 2005

Figure 26

Scapular positioning

Scapular alignment

This scapular alignment greatly increases the range of positions in which the criteria for glenohumeral stability can be met (see figure 26). Consider the arm elevated 90 degrees in the sagittal thoracic plane. This position can be achieved with the scapula protracted or retracted. If the scapula is protracted, the humerus is closely aligned with the glenoid center line. When the humerus is in this position, most of the humeroscapular muscles are oriented to compress the humeral head into the glenoid fossa. Alternatively, if the scapula is maximally retracted, the humerus is almost at right angles to the glenoid center line. In this position, the net humeral joint reaction force is directed posteriorly and may not be contained within the balance stability angle. (Bradley and Tibone, 1991; Glousman et al, 1988; Inman et al, 1994; Ozaki, 1989; Poppen and Walker, 1978; Warner et al, 1992)

Which humeroscapular position is used to achieve a given humerothoracic position is a question of habit, and training. The coordination of scapular position and glenohumeral muscle balance are important elements of the neuromuscular control of glenohumeral stability.

Atwater (Atwater, 1980) has documented that in most throwing and striking skills, the shoulder abduction angle is usually 100 degrees. Higher and lower release points are achieved by tilting the trunk rather than by increasing or decreasing the shoulder abduction angle relative to the trunk.

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