Shoulder proprioception: latent muscle reaction times

Purpose: The purpose of this study was to identify electromyographic (EMG) differences in the latent muscle reaction timing (LMRT) of the rotator cuff between trained overhead throwers and control subjects in response to sudd en internal rotation perturbation (P less than or equal to 0.05). Methods: Subjects included 15 trained overhead throwers (male intercollegiate baseba ll players) and 15 untrained subjects (males not active in competitive thro wing sports). Subjects were tested while seated, with their dominant glenoh umeral joint positioned in 90 degrees abduction/external rotation (scapular plane), their elbow flexed to 90 degrees, and their forearm placed in the perturbation device. Rotator cuff LMRT was assessed as they tried to decele rate a variably timed, sudden internal rotation force. EMG sampling (2000 H z, 2-s duration) began immediately before perturbation. Results: Trained th rowers had slower infraspinatus (P = 0.011) and teres minor (P = 0.024) LMR T and decreased supraspinatus (P = 0.001) and posterior deltoid (P = 0.0001 ) muscle activation duration compared with control subjects. Conclusions: T hese results suggest that the rotator cuff muscles of trained throwers may be downregulated in response to sudden internal rotation perturbation. Alth ough these adaptations would enable greater internal rotation velocities du ring overhead throwing, they may also contribute to glenohumeral joint path ology. The identification of changes in rotator cuff LMRT in response to su dden internal rotation perturbation suggests an area of acquired neuromuscu lar imbalance warranting consideration by those involved in the rehabilitat ion and conditioning of the overhead throwing athlete.