Anatomy and function of the shoulder
The shoulder, along with the elbow, is the link between the brain and the hand. Since human beings are bipedal, they need to be able to place and move their hands freely in space in order to hold objects, express themselves or be creative. Since the shoulder is the most flexible joint in the human body, it allows this mobility, which is at once fluid, precise and powerful. Its remarkable faculties reflect a complex anatomical structure and biomechanics.
Four joints in one
Three “true” joints and one false one:
1. Glenohumeral joint: Formed by the head of the humerus and the glenoid cavity (shoulder blade). It is the most important and largest joint and the first to move when the arm is raised. This joint, in which the head of the humerus is much larger than the glenoid, is structurally unstable. It, therefore, relies on the entire ligament system (capsule and labrum) and the tendons of the rotator cuff to prevent dislocations.
2. Acromioclavicular joint: Junction of the lateral end of the clavicle and the acromion (part of the shoulder blade). It has a limited range of motion (only moves when the arm is raised overhead), but its primary function is to attach the upper limb to the rest of the body (with the sternoclavicular joint).
3. Sternoclavicular joint: Junction of the sternum and the clavicle. It has a limited range of motion and completes the acromioclavicular joint.
4. Scapulothoracic joint: This is not a true joint but rather the gliding movement between the shoulder blade (scapula) and the thoracic cage that determines the position of the shoulder blade and, consequently, those of the glenoid and the glenohumeral joint. Depends on muscle integrity and a certain “smoothness” in the sequence of muscle contraction, often disrupted following injury.
Muscles for strength and muscles for stability
Since shoulder function involves four joints, certain muscle groups ensure the optimal positioning of the bony structures (for example, the rotator cuff muscles that position the humeral head in front of the glenoid and the muscles that move the shoulder blade), while others provide strength and power (deltoids, pectoralis major, latissimus dorsi).