Shoulder electromyography in multidirectional instability.

We studied shoulder muscle activity in multidirectional instability (MDI) and multidirectional laxity (MDL) of the shoulder, our hypothesis being that altered muscle activity plays a role in their pathogenesis. Six muscles (supraspinatus, infraspinatus, subscapularis, anterior deltoid, middle deltoid, and posterior deltoid) were investigated by use of intramuscular dual fine-wire electrodes in 7 normal shoulders, 5 MDL shoulders, and 6 MDI shoulders. Each subject performed 5 types of exercise (rotation in neutral, 45 degrees of abduction, 90 degrees of abduction, flexion/extension, and abduction/adduction) on an isokinetic muscle dynamometer at two rates, 90 degrees /s and 180 degrees /s. After filtering, rectification, and smoothing, the electromyography signal was normalized by using the peak voltage of the movement cycle. In subjects with MDI, compared with normal subjects, activity patterns of the anterior deltoid were different during rotation in neutral and 90 degrees of abduction, whereas those of the middle and posterior deltoid were different during rotation in 90 degrees of abduction. In subjects with MDL, the posterior deltoid showed increased activity compared with normal subjects during adduction. Activity patterns of the supraspinatus, infraspinatus, and subscapularis appeared similar in both groups. Dual fine-wire electromyography offers insight into the complex role of shoulder girdle muscle function in normal movement and in instability. Altered patterns of shoulder girdle muscle activity and imbalances in muscle forces support the theory that impaired coordination of shoulder girdle muscle activity and inefficiency of the dynamic stabilizers of the glenohumeral joint are involved in the etiology of MDI. Interestingly, the abnormalities are in the deltoid rather than the muscles of the rotator cuff.