The structure indicated is the deltoid muscle.
There are several muscles which act on the glenohumeral joint to produce movement at the shoulder. The deltoid is a large, powerful superficial muscle which is responsible for giving the shoulder its characteristic contour. The deltoid muscle is comprised of three sets of muscle fibres:
- Anterior fibres
- Lateral fibres
- Posterior fibres
The various groups of fibres of the deltoid muscle originate on the clavicle and scapula and converge to insert onto the deltoid tuberosity of the humerus.
The anterior fibres originate from the anterior border of the lateral third of the clavicle on its superior surface.
The lateral fibres originate from the superior aspect of the acromion process.
The posterior fibres originate from the posterior border of the spine of the scapula
The deltoid muscle is innervated by the axillary nerve and functions to abduct, flex and extend the humerus at the glenohumeral joint, depending on the rotation of the humerus and which groups of the fibres are contracting.