The synergistic action of the capsule and the shoulder muscles.

The existence of a reflex arc from the glenohumeral capsule to several muscles crossing the shoulder joint was determined in the feline model. Three branches of the axillary nerve terminating in the glenohumeral capsule were identified and electrically stimulated with supramaximal, 100-microseconds pulses using bipolar hook electrodes. Stimulation of the anterior and the inferior axillary articular nerves elicited electromyographic activity in the biceps, subscapularis, supraspinatus, and infraspinatus muscles. Stimulation of the posterior axillary articular nerve elicited electromyographic activity in the acromiodeltoid muscle. Transection of the three articular nerves just distal to their emergence from the main axillary nerve resulted in the absence of any electromyographic activity in the muscles on stimulation, confirming the afferent nature of the articular branches. The time from application of the stimulus to the appearance of a response in the muscles varied from 2.7 msec in the biceps to 3.1 msec in the supraspinatus. The existence of a reflex arc from mechanoreceptors within the glenohumeral capsule to muscles crossing the joint confirms and extends the concept of synergism between the passive (ligaments) and active (muscles) restraints of the glenohumeral joint. This provides new information in orthopaedic sciences that has direct application in modification of surgical repairs and therapeutic modalities of shoulder injuries.