The shoulder has the largest range of motion of any joint in the human body. The vast mobility of the shoulder joint allows the upper extremity to rotate up to 180 degrees in three different planes of direction. As a result, the shoulder joint is particularly susceptible to injury and highly unstable. The muscles, tendons and ligaments of the shoulder’s rotator cuff work together to compensate for this instability. When any part of this complex relationship is compromised the shoulder joint will not behave properly causing pain or loss of mobility. In order to understand the dynamic of the shoulder joint it is important to understand how the bones, muscles and ligaments work together.
The bones of the shoulder include the humerus, or upper arm bone, the scapula, or the shoulder blade, and the clavicle, or the collar bone.
Humerus – The humerus is the long upper arm bone that runs from the shoulder to the elbow. The humerus connects the scapula to the bones of the lower arm.
Scapula – The scapula connects the humerus to the clavical (collar bone). The scapula makes up the back of the shoulder girdle and is located posterolateraly to the thoratic cage.
Clavicle – The clavicle extends from the scapula to the sternum, and serves as a strut keeping the bones in position, allowing the arm to hang freely. The entire length of the clavicle is palpable in nature and is a common spot for injury.
The shoulder joint is a multiaxial ball and socket joint, similar to the hip, in which the rounded head of the humerus fits into the scapula at the joint cavity. The humerus is held in place by the numerous ligaments and tendons that make up the shoulder joint.
The shoulder joint has the dual task of strength and mobility that allows you to lift heavy objects and move your arm 360 degrees. The relationship between mobility and stability makes the shoulder joint the most complex joint in the human body. Three smaller joints comprise the entire shoulder joint:
There are many muscles that make up the shoulder and contribute to mobility.
Muscles that act on the anterior shoulder:
Serratus Anterior: stabilizes the scapula by holding it to the chest wall
Pectoralis Minor: aids the scapula in forward and downward movement
Muscles that act on the posterior shoulder:
Levator Scapulae: pulls scapula upward and bends neck laterally
Trapezius: aids in rotating scapula
Rhomboid Major: retracts and elevates the medial border of the scapula while downwardly rotating the lateral angle
Rhomboid Minor: rotates medial border of scapula while downwardly rotating the lateral angle
|Movement of the Shoulder Joint|
|Shoulder lateral rotation||0-90|
|Shoulder medial rotation||0-90|