The TuneUp Program

The deltoid muscle is named for it's triangular shape, like a delta. It has there distinct sections the anterior deltoid attaches on the clavicle, the middle deltoid on the acromion process of the scapula and the posterior fibres on the scapula spine. All three insert on the lateral side of the humerus about a third of the way down.

Because of it's shape it's able to pull in three distinct directions on the one point. While the entire muscle is responsible for abducting the humerus the anterior fibres also play a role in extension and medial rotation while the posterior fibres aid in extension and lateral rotation of the humerus.

Deltoid is a powerful muscle that contributes to most of the movements of the shoulder. Because it produces so much of the force of shoulder movements a lot of the other structures in the shoulder are active stabilsing the pectoral girdle to give it a base to pull from.

The right shoulder is abducted to varying degrees when you play your guitar and the deltoid is the primary muscle that maintains this position.