The Effects of Exercise Training on Shoulder Neuromuscular Control
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The human shoulder complex relies on the sensorimotor system to maintain stability. The sensorimotor system includes sensory feedback, control of the central nervous system and motor output. Exercise is considered an important part of shoulder rehabilitation and sports training to help improve control of the sensorimotor system. However, few studies have investigated the effect of exercise on the sensorimotor system. The first study of this dissertation explored the central control of the deltoid and rotator cuff (infraspinatus). Although both the deltoid and infraspinatus contribute to shoulder abduction, the results from this study showed that the modulation of their corticospinal excitability was affected differently by elevation angle. This could be explained by the fact that they play different roles at the shoulder: the deltoid is a prime mover while the infraspinatus is a stabilizer. The second study of this dissertation investigated scapular proprioception, which has not been assessed in previous studies. The findings of this study demonstrated that joint position sense errors of the overall shoulder joint mainly came from the glenohumeral joint. Scapular proprioception may need to be tested separately in addition to overall shoulder proprioception. In the third study, the effect of the exercise on shoulder sensorimotor system was investigated by measuring shoulder kinematics, shoulder joint position sense and cortical excitability before and after a four-week exercise training program. This protocol included strengthening and neuromuscular exercises targeting rotator cuff and scapular muscles. After the training protocol, although strength increased overall, the only observed sensorimotor adaptations were a decrease in upper trapezius activation and a decrease in the corticospinal excitability of the supraspinatus. There were no changes in other key parameters. Exercises focusing on specific muscles, combined with low-intensity closed-chain exercises, were not found to improve shoulder joint position sense or scapular kinematics. Combined with the findings of the decrease in corticospinal excitability of the supraspinatus and no change in muscle activity of the rotator cuff, it appears that while the exercises increased rotator cuff strength, these gains did not transfer to an increase in muscle activation during motion. This dissertation includes previously published co-authored material.