Anatomy of the Shoulder: Understanding the Muscles

Your shoulder contains six muscles, three bones, and three joints. The shoulder joint is the most flexible joint and is also considered one of the most complex. The way that the bones, joints, and muscles interact allows us to move our arms in every direction imaginable. However, this flexibility comes at a price. The shoulder is prone to injuries if not properly taken care of. Muscle imbalances are one of the leading causes of injury. If you’re not properly developing each of your shoulder muscles, you are putting yourself at risk for long term injuries or pain.

Bones of the Shoulder

Shoulder joint highlighting the humerus, scapula, and clavicle1.

The shoulder is composed of the collarbone (clavicle), the shoulder blade (scapula), and the upper arm bone (humerus). Together these bones form what’s known as the shoulder girdle.

The shoulder blade, collarbone, and upper arm bone form three joints within the shoulder girdle.

  1. The shoulder joint which connects the upper arm to the shoulder blade.
  2. The sternoclavicular joint which connects the sternum to the clavicle.
  3. The acromioclavicular joint which connects the bone at the top of the shoulder (acromion) to the collarbone (clavicle).

Almost all the movements of your shoulder involve more than one of these joints.

Muscles of the Shoulder

The muscles of the shoulder can be classified as being either extrinsic or intrinsic. The extrinsic muscles start from the torso and attach to one of the shoulder bones. They make up the muscles of your back such as your Lats and traps. The intrinsic shoulder muscles start from the scapula or the clavicle and attach to the humerus. This article will cover the intrinsic muscles of your shoulder.

There are six intrinsic shoulder muscles: the deltoid, teres major, and the four rotator cuff muscles. The supraspinatus, infraspinatus, subscapularis, and teres minor muscles and their tendons make up the rotator cuff.

Deltoid

Animation
Anterior fibers of the deltoid is in red. Posterior fibers of the deltoid is in blue. Middle fibers of the deltoid is in green2.

When we think of the shoulder muscle, immediately the deltoid comes to mind. The deltoid is the most predominant muscle of our shoulders and is responsible for it’s round shape. It has three sections: The anterior section, posterior section, and the middle section.

The Anterior fibers are responsible for flexing the shoulder and rotating your arm towards your body. An example of shoulder flexion is when you move your arms forwards. The posterior fibers are responsible for extension of the shoulders and rotation of the arms away from your body. An example of shoulder extension is when you bring your arms behind you. The middle fibers are responsible for lifting your arms to the side.

Teres Major

Teres major.PNG
Teres major view from the back and highlighted in red3.

The teres major is a smaller and lesser known muscles of the shoulder. However, it plays an important role for the movement of your arms. This muscle originates on the back of your scapula and attaches to upper inside of the humerus bone. This muscle is responsible for the movement of your arm towards your body, rotation of your arm towards your body, and moving your arm backwards. Additionally, it helps you lower your shoulder.

Supraspinatus

Supraspinatus.PNG
View of the supraspinatus muscle from the back highlited in red4.

This is a relatively small muscle that is located on your upper back. This muscle starts on the upper portion of your scapula and attaches to the upper portion of your humerus bone. Although it is small, it is crucial to preventing shoulder injuries. This muscle works with the middle deltoid and helps you raise your arms to the side. Additionally, it stabilizes the humerus preventing it from dislocating.

Infraspinatus

Infraspinatus.PNG
A view of the infraspinatus muscle from the back highlighted in red5.

This muscle is a very small but thick. It is located on the upper back. It starts from the lower section of the scapula and attaches to the upper portion of the humerus. Although this muscle is small, it plays a big role in injury prevention. This muscle helps you rotate your arm away from your body. Additionally, this muscle helps prevent injuries by stabilizing your shoulder joint.

Subscapularis

Subscapularis muscle frontal.png
THe subscapularis muscle highlighted in red from the front6.

Unlike the previously discussed muscles of the rotator cuff, this muscle is located on the inside of the scapula. It is a flat muscle that starts on the inside edge of the scapula and attaches to the upper inside of the humerus. It is responsible for rotating your arm towards your body and bringing your arm in towards the center of your body. Additionally, this muscle stabilizes your shoulder joint.

Teres Minor

Teres minor muscle back3.png
Teres minor muscle highlighted in red7.

This is a very long and thin muscle that is located on your upper back. It starts from the back of your scapula and attaches to the upper inside of your humerus. Although this muscle is small, it plays a big role in the movement of your arms. It is responsible for the rotation of your arm away from your body. Additionally, this muscle stabilizes your humerus within the shoulder joint.

Conclusion

The shoulder is one of the most complex areas of the body. It is composed of six muscles, three joints, and three bones. The deltoids are the most commonly known muscle group and are responsible for the roundness of your shoulders. The three sections of the deltoid are responsible for moving your arms forward, backward, and to the side. The teres major is responsible for moving your arm forward and rotation of your arm towards your body. Additionally, this muscle helps you lower your shoulders. The rotator cuff is composed of four muscles that help with the stabilization of your shoulder joint. Furthermore, they help with the rotation of your arm both inwards and outwards.

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