Excitability of the infraspinatus, but not the middle deltoid, is affected by shoulder elevation angle

Although both the rotator cuff and deltoid muscle serve as shoulder abductors, they play different roles in shoulder movement. While the deltoid is a primary abductor, the rotator cuff is a stabilizer. They have different anatomic structures for force production and demonstrate different neuromuscular control at different shoulder angles, as measured by electromyographic activity. Corticospinal excitability may be associated with different neuromuscular control of the deltoid and rotator cuff at different angles. The purpose of this study was to investigate how shoulder joint position influences the corticospinal excitability of the deltoid and rotator cuff muscles. Transcranial magnetic stimulation was used to measure the corticospinal excitability of the middle deltoid and infraspinatus at 0° and 90° of arm elevation. Three parameters, a plateau value, exponential parameter, and threshold, were calculated from the input–output curve of the corticospinal pathway. The plateau value of the infraspinatus was significantly higher at 90° of arm elevation, while there is no difference in the excitability in the middle deltoid between elevation angles. The plateau value of the middle deltoid at 90° was 5 % lower than that at 0°, but the plateau value of infraspinatus at 90° was 55 % higher than that at 0°. This suggests that the modulation of excitability varies with shoulder angle and reveals different neurological mechanism for the roles of the deltoid and rotator cuff.