Assessment of muscle -joint complex stiffness and reflex latencies in anterior glenohumeral instability patients

Dynamic stability is reported to arise from proprioception and neuromotor control over the skeletal muscles crossing the shoulder joint. While proprioceptive deficits have been demonstrated in shoulders diagnosed with glenohumeral instability, questions remain as to how the neuromotor mechanisms are affected. The purpose of this study was to assess both the intrinsic and extrinsic components of stability as measured with muscle joint complex stiffness, muscle reflex latency, and electromechanical delay in unstable shoulders during an apprehension perturbation. Nine subjects diagnosed with traumatic, recurrent, anterior instability were matched with normal subjects. Nine shoulders were utilized for each group. Subjects were seated with their upper extremity secured in a position of 90° elevation in the scapular plane with external rotation, in a modified isokinetic dynamometer. The apprehension perturbation consisted of an external humeral rotation collision force applied to the anterior forearm at 180°/sec by an isokinetic dynamometer. Muscle contraction levels during perturbation included a relaxation state (0%), 20%, and 50% of a maximum isometric internal rotation contraction. Fine-wire electromyography (EMG) measured rotator cuff muscle reflex latency while surface EMG assessed the muscle reflex latency of the primary humeral movers. A compression load cell was mounted on the isokinetic lever and utilized for calculation of electromechanical delay and muscle joint complex stiffness. Six perturbation trials were performed at each level of contraction. Statistical analysis indicated that no differences existed between the unstable and stable groups with respect to muscle reflex latency, electromechanical delay, and muscle joint complex stiffness under any level of contraction. Faster muscle reflex latencies were present with the latissimus dorsi, upper subscapularis, and lower subscapularis muscles when comparing latencies between the 0% and 20% and 0% and 50% contraction levels. Decreases in electromechanical delay and increases in muscle joint complex stiffness manifested as muscle contraction increased from 0% to 20% and 20% to 50% respectively. Results from this study indicate that factors other than muscle reflex latency, electromechanical delay, and muscle joint complex stiffness may contribute to symptoms of shoulder instability. Additionally, the results of this study demonstrate the importance of muscle contraction in augmenting joint stability.