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Infraspinatus Muscle: Definition, Function & Innervation

Instructor: Dan Washmuth

Dan has taught college Nutrition, Anatomy, Physiology, and Sports Nutrition courses and has a master's degree in Dietetics & Nutrition.

The infraspinatus muscle plays a very important role in the mobility and stability of the shoulder joint. This lesson will define this important shoulder muscle and describe its function and innervation.

The Hitchhiking Muscle

Have you ever seen a hitchhiker on the side of the road with their arm rotated externally out to their side and their thumb sticking straight up? Do you know what muscle hitchhikers use to rotate their arms out to the side? It is the infraspinatus muscle, one of the many muscles that make up the shoulder joint.


The infraspinatus in use.
hitchhiker


What is the Infraspinatus Muscle?

The infraspinatus muscle is a triangular-shaped muscle that is located at the back of the shoulder joint, on the back surface of the scapula (shoulder blade). It attaches to the top of the humerus (long bone of the upper arm). This muscle is one of the four muscles that form the rotator cuff, which functions to stabilize the shoulder joint.

The infraspinatus gets its name because this muscle is located in the infraspinous fossa of the scapula. The infraspinous fossa is the concave surface at the back of the scapula.


The infraspinatus muscle is located at the back of the shoulder.
infraspinatus


Function of the Infraspinatus Muscle

Like all rotator cuff muscles, the infraspinatus functions to stabilize the shoulder joint. Specifically, the infraspinatus functions to keep the head of the humerus in its correct position against the scapula while the humerus is moved throughout the shoulder joint.

Additionally, the infraspinatus functions to laterally (externally) rotate the humerus, which involves the outward turning of the upper arm. To perform lateral rotation of the humerus, you can simply bend your elbows at a 90-degree angle, then rotate your upper arms so your hands rotate out to the sides of your body. This is the same motion that a hitchhiker must perform in order to move their arm out to their side with their thumb sticking up.

Infraspinatus Innervation

The infraspinatus muscle receives electrical impulses from the brain through the suprascapular nerve. The suprascapular nerve is a nerve that consists of nerve fibers from the fifth and sixth cervical nerve (C5 and C6). The cervical nerves are nerves that originate from the neck region of the spinal cord.

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