Deltoid Muscle

The deltoid is a thick, triangular shoulder muscle. It gets its name because of its similar shape to the Greek letter ‘delta’ (Δ). The muscle has a wide origin spanning the clavicle, acromion and spine of scapula. It passes inferiorly surrounding the glenohumeral joint on all sides and inserts onto the humerus. 

The deltoid has three functionally and anatomically distinct parts. The acromial part, sometimes also known as the middle or  central, is the largest and the strongest. It is a multipennate muscle. It arises as four intramuscular septa, which interdigitate with the three tendons at the insertion site (one each for the anterior, posterior and middle parts). The four septa are connected by short, strong muscle fibres.