Changes in corticospinal excitability of shoulder muscles across functional glenohumeral joint positions.

The shoulder is a complex region of the body that can be prone to injury associated with awkward postures due to a broad range of motion and relatively low stability. A better understanding of how arm orientation influences neural drive (excitability) to muscles surrounding the glenohumeral joint can provide insight into recruitment strategies. The purpose of this study was to determine the influence of glenohumeral joint angle on excitability of the muscles surrounding the shoulder complex. Ten participants underwent transcranial magnetic stimulation in five randomized joint angles: neutral, 45°, and 90° of elevation in shoulder flexion and abduction. Surface electromyography was collected from eight muscles of the dominant arm: biceps brachii, triceps brachii, anterior, middle, and posterior deltoid, supraspinatus, infraspinatus, and upper trapezius. Motor evoked potentials (MEPs) were elicited as stimulus response curves (SRC) in the resting muscles, with intensities of 85/100/115/130/145/160% of resting motor threshold. Results of this work revealed an angle and stimulus intensity interaction; 90° of shoulder abduction elicited the greatest MEP amplitudes across all intensities, whereas neutral resulted in the lowest MEP amplitude. There were main effects of joint angle and muscles, as supraspinatus resulted in the largest variability across neutral, 45° flexion, and 45° abduction as compared with deltoid muscles. This work highlights that changes in glenohumeral joint angle at the shoulder affect corticospinal excitability while at rest. Understanding how posture affects excitability for a range of muscles that surround the shoulder complex could have useful implications for task design and injury prevention. This work reports the influence of glenohumeral joint angle on resting corticospinal excitability of muscles of the shoulder complex. This work demonstrated that 90° of shoulder abduction resulted in the greatest excitability across all muscles, whereas the neutral position resulted in the lowest MEP amplitude. These findings provide evidence that changes in glenohumeral joint angle affect corticospinal excitability while at rest.