The Effects of Exercise Training on Shoulder Neuromuscular Control

Significance

The human shoulder complex sacrifices stability in exchange for a large range of motion necessary for hand manipulation. Due to the inherent instability of the bony congruence and ligament constraint, dynamic control of the muscles plays an important role in stabilizing the shoulder during motion. The rotator cuff muscles serve as the chief stabilizer of the shoulder joint, while the deltoid provides most of the torque necessary for motion. Therefore, the coordination of the deltoid and the rotator cuff muscles is essential for smooth and efficient shoulder function. Impairment in the neuromuscular control of these muscles may to lead to injury and pain. Consequently, rehabilitation programs aim to restore the normal neuromuscular control of these shoulder muscles in order to help decrease pain and improve shoulder function. Although rehabilitation programs have demonstrated positive effects on pain decrease and functional improvement, it is still unknown if they serve to improve the neuromuscular control of the shoulder muscles.

While the deltoid provides most of the torque necessary to elevation the shoulder, it can also produce a superiorly directly force on the glenoid. This shear force tends to pull the humeral head superiorly, which can result in a decrease in the subacromial space. When the rotator cuff muscles function properly, the line of action of the rotator cuff muscles result in a centering force, which can help maintain the humeral head in the center of the glenoid fossa. However, it has been hypothesized that if the rotator cuff muscles are not able to produce sufficient force during arm movement, the superior shear force produced by the deltoid results in humeral head superior translation. This abnormal displacement can result in impingement of subacromial tissues and ultimately leading to tissue injury and pain.

Traditionally, shoulder rehabilitation programs focus on strengthening the rotator cuff muscles by using shoulder movement in which the rotator cuff muscles show high muscle activity. However, although these activities lead to a strengthening of the rotator cuff, what is unknown is whether the repetitive stress imposed on the rotator cuff results in neuromuscular adaptations that will help counteract the deltoid shear force.

This stress of rehabilitation may induce the neurological and physiologic changes in neuromuscular structures and thus alters the neuromuscular control of the entire shoulder complex. Kinematics and electromyographic (EMG) measurements have been widely used to study neuromuscular control. While kinematic measures show movement patterns, EMG demonstrates the timing, sequence, and magnitude of the muscle firing. Abnormal kinematics and EMG patterns have been demonstrated in patients with shoulder impingement. Measuring parameters of neuromuscular control can lead to a better understanding of the underlying mechanism of shoulder exercise training and how the neuromuscular structures adapt to the stress of the exercise training. More specifically, these assessments can be used to assess whether a rehabilitation program results in a positive adaptation that can help the shoulder muscles handle the stress from activities such as overhead sports activities and carrying or lifting heavy objects. Ultimately, this could be used to design more efficient training program for athletes and effective rehabilitation program for patients with shoulder injuries.