Mechanisms of Modulation of Automatic Scapulothoracic Muscle Contraction Timings

Erected posture provides humans a large shoulder mobility that requires complex automatic muscle synergies to accomplish joint stability needs. This is evident in shoulder abduction, wherein the voluntary activation of glenohumeral muscles is coupled with an automatic recruitment of scapulothoracic muscles. Here, we investigated whether volitional modification of the scapular position, and dynamic scapular elevation, modulate the contraction timing of five shoulder muscles (middle deltoid, upper, middle and lower fiber of the trapezius, serratus anterior) during shoulder abduction. The results show matched contraction timings of the deltoid and upper trapezius across the scapular positions, whereas the contraction timings of the middle and lower fibers of the trapezius change secondary to the scapular position. These results might reflect different central strategies to coordinate the automatic sequences of contraction of the scapulothoracic muscles. This suggest a flexible and adaptable predisposition of the motor control system in exploring alternative solutions to accomplish the functional movement needs, such as the fulfillment of unconstrained movements. Intriguingly, the shoulder abduction may represent a powerful, non-invasive, and straightforward tool to deepen the understanding of the neural basis underlying the voluntary motor command modulation of the out-of-volition automatic muscle contractions.