The shoulder is comprised of bones, joints, ligaments, muscles and tendons.
Bones: clavicle (referred to as the collar bone), scapula (shoulder blade) and the humerus (upper arm bone).
The shoulder has two joints that work together to allow arm movement. The bones are joined by tendons, ligaments, and muscles (known as soft tissues).
Joints: (i) acromioclavicular (AC) joint allows the clavicle and the acromion (the part of the scapula that forms the point on the shoulder). AC joint lets us raise the arm above the head.
(ii) glenohumeral (shoulder joint) is the ball and socket type joint. The ball is the rounded upper end of the humerus, The socket is the concave part of the scapula (called the glenoid). The glenohumeral joint allows the arm to move in a circular motion + towards/away from the body. The labrum is a piece of cartilage that acts as a cushion in this ball/socket joint. (iii) sternoclavicular joint.
Two sacks called bursa are located in the shoulder. Bursa secrete lubricant that reduces friction between the joint’s moving parts.
Muscles: the deltoids (anterior deltoid, posterior deltoid and lateral deltoid) are the major shoulder muscles.
- Anterior Deltoid is located on the front, bordering with the chest muscles. Anterior deltoids provide shoulder flexion (lets you raise your arms over your head and in front of you).
- Posterior Deltoid is on the back of the shoulder and provides shoulder extension (lets you bring your shoulder blades together.
- Lateral Deltoid is on the outside portion of the shoulder and providesshoulder abduction (lets you raise your arms upwards at your sides).
Not to be overlooked, the rotator cuff is comprised of four muscles (supraspinatus, infraspinatus, subscapularis, and teres minor) that pull the humerus into the scapula and stabilize the glenohumeral joint which helps us with arm rotation. As is indicated by the name, these muscles give us the ability to rotate our shoulders and upper arms in a wide variety of motions.