Mechanics of Glenohumeral Instability.

Last updated Friday, February 04, 2005

Figure 12

Figure 13

Figure 14

The net humeral joint reaction force

Neuromuscular training

Each active muscle generates a force whose direction is determined by the effective origin and insertion of that muscle (see figure 12). The neural control of the magnitude of these muscle forces provides the mechanism by which the direction of the net humeral joint reaction force is controlled. For example, by increasing the force of contraction of a muscle whose force direction is close to the glenoid center line, the direction of the net humeral joint reaction force can be aligned more closely with the glenoid fossa (see figure 13).

The elements of the rotator cuff are well positioned to contribute to this muscle balance. (Basmajian and DeLuca, 1985; Bassett et al, 1990; Blasier et al, 1992; Cain et al, 1987; Harryman et al, 1996, Itoi et al, 1993; Itoi et al, 1994; Karlsson and Peterson, 1992; Pagnani et al, 1995; Perry and Glousman, 1989; Rodosky et al, 1994; Sarrafian, 1983; Van der Helm, 1994; Van der Helm et al, 1992; Veeger et al., 1991; Wulker et al, 1995)

Strengthening and neuromuscular training help optimize the neuromuscular control of the net humeral joint reaction force. Conversely, the net humeral joint reaction force is difficult to optimize when muscle control is impaired by injury, disuse, contracture, paralysis, loss of coordination, or tendon defects (see figure 14).

Neuromuscular training may be guided by proprioceptors in the labrum and ligaments (Guanche et al, 1995; Hashimoto et al, 1994; Jerosch et al, 1995; Vangsness et al, 1995) which may help guide neuromuscular training. Blasier et al (Blasier et al, 1994) and Kronberg et al (Kronberget al, 1991) showed that individuals with generalized joint laxity have less acute proprioception and altered muscle activation. Zuckerman et al demonstrated that motion and position sense are compromised in the presence of traumatic anterior instability and restored at one year after surgical reconstruction. (Zuckerman et al., 1996)

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