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The Synergistic Action of the Capsule and the Shoulder Muscles

Bioengineering Laboratory, Department of Orthopaedic Surgery, Louisiana State University Medical Center, New Orleans, Louisiana

Theodore Knatt, MD

Bioengineering Laboratory, Department of Orthopaedic Surgery, Louisiana State University Medical Center, New Orleans, Louisiana

Moshe Solomonow, PhD

Bioengineering Laboratory, Department of Orthopaedic Surgery, Louisiana State University Medical Center, New Orleans, Louisiana

Yun Lu, MD

Bioengineering Laboratory, Department of Orthopaedic Surgery, Louisiana State University Medical Center, New Orleans, Louisiana

Richard Baratta, PhD

Bioengineering Laboratory, Department of Orthopaedic Surgery, Louisiana State University Medical Center, New Orleans, Louisiana

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 cap sule were identified and electrically stimulated with su pramaximal, 100-µsec pulses using bipolar hook elec trodes. Stimulation of the anterior and the inferior axillary articular nerves elicited electromyographic ac tivity in the biceps, subscapularis, supraspinatus, and infraspinatus muscles. Stimulation of the posterior ax illary 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 elec tromyographic activity in the muscles on stimulation, confirming the afferent nature of the articular branches. The time from application of the stimulus to the appear ance 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.

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