The shoulder joint
The shoulder joint is a ball and socket joint, with the head of the humerus fitting into a very shallow cavity on the scapula, called the glenoid fossa. The shoulder joint is the most mobile joint in the body, but also one of the most unstable because the shallow cavity gives little support to the head of the humerus. Stability has to be provided by ligaments and muscles, and a total of nine muscles cross each shoulder joint to insert on the humerus. All of these muscles have their origin on the shoulder girdle.
Only three of the nine muscles act as prime movers for arm movements – pectoralis major, latissimus dorsi and the deltoid muscles. The other six are used as synergists or fixators.
Four of the nine muscles, the supraspinatus, infraspinatus, teres minor and subscapularis muscles, are collectively known as rotator cuff muscles. Their main function is to reinforce the shoulder joint capsule to help prevent dislocation of the humerus. They also, however, act as synergists for angular or rotational movements of the arm.
The last two muscles, the teres major and coracobrachialis, cross the shoulder joint, but do not reinforce it.
Movements possible at the shoulder joint include flexion, extension, horizontal flexion, horizontal extension, abduction, adduction, medial rotation, lateral rotation and circumduction.
Muscles that have their origins in the anterior (front of) shoulder joint tend to flex the arm (pectoralis major, coracobrachialis and anterior fibres of the deltoid). Pectoralis major is the muscle which acts as prime mover in shoulder flexion.The biceps brachii assist this movement.
Muscles that have their origins in the posterior (back of) shoulder joint extend the arm. These muscles include the latissimus dorsi and posterior fibres of the deltoids, with both acting as the prime mover. Teres major also assists this action. Pectoralis major and latissimus dorsi act as antagonists.
The middle region of the deltoid muscle is the prime mover for arm abduction. The pectoralis major acts as an antagonist to the middle deltoid anteriorly, whilst the latissimus dorsi acts as the antagonist posteriorly.
Depending on their location and their points of insertion, the various muscles that act on the upper arm also assist medial and lateral rotation of the shoulder joint. The interactions of these nine muscles are complex, with each muscle contributing to more than one movement.