The shoulder joint

The shoulder joint is a ball and also socket joint, v the head the the humerus fitting right into a really shallow cavity ~ above the scapula, called the glenoid fossa. The shoulder joint is the most mobile share in the body, but also one of the most unstable since the shallow cavity gives tiny support come the head that the humerus. Stability needs to be listed by ligaments and muscles, and a complete of nine muscles cross every shoulder share to insert top top the humerus. All of these muscles have actually their beginning on the shoulder girdle.

You are watching: The deltoid is a prime mover of the arm that acts in adduction.

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Only 3 of the ripe muscles act as prime movers for arm movements – pectoralis major, latissimus dorsi and the deltoid muscles. The other six are offered as synergists or fixators.

Four of the nine muscles, the supraspinatus, infraspinatus, teres minor and also subscapularis muscles, space couchsurfingcook.comllectively known as rotator cuff muscles. Their main role is to reinforce the shoulder joint capsule to help prevent dislocation of the humerus. Lock also, however, act as synergists because that angular or rotational movements of the arm.

The last 2 muscles, the teres major and also couchsurfingcook.comracouchsurfingcook.combrachialis, overcome the shoulder joint, however do not reinforce it.

Movements feasible at the shoulder joint include flexion, extension, horizontal flexion, horizontal extension, abduction, adduction, medial rotation, lateral rotation and circumduction.


Muscles that have their beginnings in the anterior (front of) shoulder joint often tend to flex the arm (pectoralis major, couchsurfingcook.comracouchsurfingcook.combrachialis and also anterior fibres of the deltoid). Pectoralis major is the muscle which acts together prime mover in shoulder flexion.The biceps brachii assist this movement.

Muscles that have their origins in the posterior (back of) shoulder joint prolong the arm. These muscles include the latissimus dorsi and posterior fibres that the deltoids, v both acting as the element mover. Teres major also assists this action. Pectoralis significant and latissimus dorsi act together antagonists.

The middle an ar of the deltoid muscle is the element mover for arm abduction. The pectoralis significant acts as an antagonist to the middle deltoid anteriorly, whilst the latissimus dorsi acts as the antagonist posteriorly.

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Depending on your location and their clues of insertion, the various muscles the act top top the upper arm additionally assist medial and lateral rotation that the shoulder joint. The interactions of these nine muscles are couchsurfingcook.commplex, with each muscle couchsurfingcook.comntributing to an ext than one movement.