The infraspinatus muscle is a thick, triangular muscle.
The infraspinatus performs a number of actions on the shoulder and humerus:
- It is the major external rotator of the shoulder.
- Working with the teres minor, subscapularis, and the deltoids, the primary function of the infraspinatus is extension, horizontal (transverse) extension and external rotation of the humerus at the shoulder joint.
- When the arm is fixed, it abducts] the inferior angle of the scapula.
- The infraspinatus and teres minor rotate the head of the humerus outward (external, or lateral, rotation); they also assist in carrying the arm backward (extension of the glenohumeral joint).
- It also helps to stabilise the shoulder joint by keeping the head of the humerus firmly pressed medially against the scapula.
- Additionally, the infraspinatus reinforces the capsule of the shoulder joint.
Figure 1. Infraspinatus muscle.
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