The anatomy of the shoulder joint allows for an impressive range of motion and utility. The combination of the glenohumeral and acromioclavicular joints allow us to flex, extend & abduct, adduct, rotate and circumvent our arms. This complex joint deftly balances our upper bodies' need for freedom of motion and joint stability through an astonishing arrangement of connective tissues, muscles and bony structure.
The Glenohumeral Joint
The two major bones of the glenohumeral joint are the scapula (the shoulder blade) and the humerus (the upper arm bone). The rounded end or head of the humerus sits in a shallow depression on the scapula called the glenoid fossa. This union of the rather large humeral head and relatively shallow glenoid fossa create a ball and socket joint that is extremely flexible. The glenohumeral joint gets the stability it needs while maintaining its impressive range of motion by relaying on connective tissue to support the joint. This network of supporting structures includes:
- The labrum - ring of cartilage that surrounds the glenoid fossa and extends the socket while keeping it flexible.
- The four muscles of the rotator cuff - the Teres minor, Infraspinatus, Supraspinatus and Subscapularis wrap around the scapula and the humerus. They provide stability to the joint and are responsible for rotating the arm.
The Acromioclavicular Joint
The lateral end of the clavicle (collarbone) and a bony projection of the scapula called the acromion meet at the highest point of the shoulder to form the acromioclavicular joint or the AC joint. This is a gliding joint that is held together by an extensive network of ligaments and muscles. The AC joint gives the scapula a greater degree of motion which makes it possible to lift the arm overhead.
The Muscles of the Shoulder Joint
The shoulder joint gets its impressive mobility by sacrificing stability. The joint is largely dependent on muscles and ligaments to maintain its integrity. The supraspinatus, infraspinatus, subscapularis, and teres minor are the four muscles of the rotator cuff. Each of these four muscles originates on the scapula and fixes itself onto the head of the humerus. Many muscles surrounding the shoulder work with the rotator cuff to rotate, abduct, and adduct the humerus. The deltoids coracobrachialis, serratus anterior, pectoralis major, pectoralis minor, latissimus dorsi, trapezius, rhomboid major, and levator scapulae muscles all work with the rotator cuff to move the arm.
The anatomy of the shoulder gives it the freedom to be the most mobile joint in all of the body. It's a complex and versatile structure.
Erika Volk is a certified Personal Trainer and Nutrition Coach. She specializes in TRX training, fitness travel, and Online weight loss and fitness coaching. She trains clients exclusively online and travels the world. Visit her website at erikavolkfitness.com to learn more about TRX Training and staying fit while traveling.
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