Understanding Shoulder Muscle Anatomy

The human shoulder is a ball-in-socket joint and in turn, is the most mobile joint in the body. This mobility allows you to move through a tremendous range of motion in a variety of planes. 

Commonly thought of as a single joint, the shoulder is actually made up of two separate joints- the glenohumeral and acromioclavicular joints


Anatomy of shoulder with muscles of the shoulder joint.  Labels for ligaments, bursae, muscles, and joints in the shoulder complex.Anatomy of the Shoulder

These two joints work together at the arm to allow the shoulder to move in a large circle and to rotate around its axis. This extensive range of motion makes the shoulder joint fairly unstable. 

This instability is countered by the strength of the rotator cuff muscles, tendons, ligaments, and the glenoid labrum. The bones of the shoulder consist of the humerus (upper arm bone), the scapula (shoulder blade), and the clavicle (collar bone). The shoulder’s relationship with the scapula allows for functional movement of the shoulder blade. Let's dive into the shoulder anatomy. 

Sometimes it is easier to understand anatomy when you look at a visual representation. Click here to watch an anatomy video about the shoulder joint anatomy.

Anatomy of Shoulder and Shoulder Joints

The bones of the shoulder come together with ligaments to allow movement and control.Bones in the Shoulder

Scapula

The scapula is a triangular-shaped bone that functions mainly as a site for muscular attachment. Four rotator cuff muscles that act on the shoulder begin at the scapula. 

These muscles are the supraspinatus, infraspinatus, Teres minor, and subscapularis. In addition, the trapezius, serratus anterior, rhomboids, and Levator scapulae insert on the scapula and are responsible for scapular mobility and stability. 

Glenoid Fossa

The glenoid fossa is the socket of the shoulder joint. This part of the shoulder is a ball and socket joint.  The head of the Humerus fits into the Glenoid fossa, though it is a shallow socket. 

Clavicle

The clavicle is an S-shaped bone that forms the front portion of the shoulder girdle and keeps the arm away from the trunk, allowing it to move freely. The clavicle consists of two joint articulations: the sternoclavicular joint and the acromioclavicular joint.  

The sternoclavicular joint is the only skeletal connection between the axial (main) skeleton and the upper extremity. Furthermore, the clavicle provides protection to the important underlying veins, arteries, and nerves that run in the front and back of the structure.

Humeral Head

The top portion of the humerus is referred to as the humeral head. The humeral head articulates with the glenoid fossa of the scapula. However, only 25% of the humeral head surface makes contact with the glenoid surface at any point in time. The glenoid labrum, a fibrocartilaginous ring, attaches to the outer room of the glenoid fossa and provides additional depth and stability.

Labrum

The glenoid labrum is a ring composed of mostly dense fibrous tissue. The average depth of the glenoid fossa is 2.5 mm, however, the labrum helps to increase this depth. Although the labrum increases the depth and volume of the glenoid fossa, it does not seem to increase the stability of the glenohumeral joint.


Ligaments in the Shoulder

  • Coracoclavicular Ligaments

The conoid and trapezoid ligaments make up the coracoclavicular ligaments. These ligaments are the primary restraint for upward and backward movement of the clavicle. 

  • Glenohumeral Ligaments

There are three glenohumeral ligaments: The superior glenohumeral ligament, middle glenohumeral ligament, and inferior glenohumeral ligament. These ligaments provide stability to the glenohumeral joint and protect from numerous translations of the humerus within the glenoid. 

  • Coracohumeral Ligament

The coracohumeral ligament originates on the base and outside the border of the coracoid process of the scapula and attaches to a landmark on the humerus called the greater tuberosity. While the function of this ligament is not fully understood, it appears to perform a suspensory function to the humeral head.

Shoulder Muscle Injuries: Causes and Treatment

Shoulder anatomy. There are several soft tissue structures in the shoulder that can become injured. Soft Tissue in the Shoulder

The most commonly injured muscles of the shoulder at the Rotator Cuff Muscles.  The Rotator Cuff is made up of 4 muscle including the:

 The muscles and tendons of the rotator cuff form a cover around the anterior, superior, and posterior humeral head and glenoid cavity of the shoulder by compressing the gleno-humeral joint. In addition to stabilization of the joint, the rotator cuff provides the shoulder with tremendous mobility.  Strained rotator cuff muscles are very common in sports.  If you want to learn more about the rotator cuff, please watch this rotator cuff injuries video.

The initial treatment for a rotator cuff strain is ice and rest.  Modalities can certainly relieve the pain.  If the pain or inability of the shoulder last for over 3 days you should see a doctor.  

  • Subacromial Bursa/Subdeltoid Bursa

The subacromial bursa lies on the top portion of the supraspinatus tendon. The bursa is a fluid-filled sac and acts to cushion and reduce friction during motion between the overlying bone of the acromion and the soft rotator cuff muscles below. It often extends out to be continuous with the sub-deltoid bursa.  Bursitis of the shoulder is treated again with modalities such as ice, heat and rest. 

Shoulder Exercises

Here is a helpful flyer of shoulder range of motion exercises.

Watch this VIDEO on why shoulder pain can't wait for treatment or read this article on the anatomy of shoulder problems. 

Posterior Shoulder Anatomy

Image of the muscles of the back of the shoulder or posterior shoulder anatomy.  The infraspinatus and teres minor are visible.Posterior Shoulder Anatomy

The muscles in the back of the shoulder include the:

  • Posterior Deltoid
  • Infraspinatus
  • Teres Minor

These muscles work to extend the arm and rotate the shoulder outward or externally.  The layer deeper to the muscles includes the labrum and joint capsule. These structures help to stabilize the shoulder from the back.

Helpful Shoulder Tip: If you have recently had a shoulder surgery or injury, please watch our video on How to Properly Put on an UltraSling 4 or Dressing the Upper Body After an Injury or Surgery

If you would like to learn more about the anatomy of the shoulder, go to: 5 Shoulder Symptoms You Should Not Ignore. 

By: Tim Wall MS, ATC 

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