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Assessment of the Infraspinatus Spinal Stretch Reflex in the Normal, Athletic, and Multidirectionally Unstable Shoulder

Wayne K. Augé, II, MD^, and David S. Morrison, MD

Center for Orthopaedic and Sports Performance Research; The Northern New Mexico Orthopaedic Center; Section of Orthopaedic Surgery, Españesbyterian Medical Center; and Public Health Service, United States Department of Health and Human Services, Santa Fe, New Mexico
Southern California Center for Sports Medicine and Section of Orthopaedic Surgery, Long Beach Memorial Medical Center, Long Beach, California

Presented at the interim meeting of the AOSSM, New Orleans, Louisiana, March 1998.

Address correspondence and reprint requests to Wayne K. Augé II, MD, 936 Vista Jemez Court, Santa Fe, New Mexico 87505

To examine neural aspects of motor control in the glenohumeral joint, this study evaluates utilization of an innate spinal segmental pathway, the spinal stretch reflex, as an investigational tool that reflects neural circuitry. The purpose of this study was to determine if this reflex could be evoked from the infraspinatus muscle, if the testing apparatus and protocol for elicitation were reliable, and if the reflex response varies between groups of subjects and therefore could be useful clinically. These reflex characteristics were evaluated in the infraspinatus muscle, since rotator cuff muscle activity in subjects with glenohumeral instability exhibits differences in electromyographic activity and coordination patterns, implicating its role in dynamic stability. Normal shoulders were compared with athletic shoulders and shoulders with multidirectional instability. The spinal stretch reflex was elicited in a controlled and reliable manner. Shoulders with multidirectional instability exhibited a more-prominent spinal stretch reflex response than normal shoulders, whereas athletic shoulders exhibited a more-quiescent spinal stretch reflex response. As the spinal stretch reflex probably plays a role in motor control, variation in this reflex profile may reflect some differences in development that contribute to the variable expression of dynamic glenohumeral stability. This study suggests that the spinal stretch reflex profile may be a useful clinical tool to assist in discriminating between the normal and pathologic state. This information may also be useful in the evaluation of new treatment approaches exploiting spinal cord plasticity and spinal stretch reflex mutability through neuromuscular training.

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