A thick pennate muscle responsible for the roundness of the shoulder. The anterior deltoid originates from the outer third of the clavicle; the middle deltoid originates from the top of the acromion process, and the posterior deltoid originates from the scapular spine. All parts of the deltoid insert onto the deltoid tuberosity of the humerus. The deltoids take part in all movements of the upper arm. They are prime movers of arm abduction and also act as antagonists of the pectoralis major and latissimus dorsi, which adduct the arm. The primary actions about the shoulder of the anterior fibres of the deltoids are flexion and horizontal adduction; those of the middle fibres are abduction and horizontal abduction; and those of the posterior fibres are extension and horizontal abduction. The deltoids are particularly active during rhythmic swinging movements of the arms during walking.
Subjects: Sports and Exercise Medicine.