Final answer:
The infraspinatus muscle is responsible for the lateral rotation of the humerus, which moves the anterior surface of the arm away from the midline of the body and assists in arm abduction.
Step-by-step explanation:
The infraspinatus muscle, located inferior to the spine of the scapula, is a member of the rotator cuff (musculotendinous cuff), which plays a critical role in stabilizing and moving the humerus. When the infraspinatus contracts, it primarily laterally rotates the humerus. Lateral rotation means that the anterior surface of the arm moves away from the midline of the body, which is the opposite of medial (internal) rotation. The infraspinatus also assists in arm abduction when the arm is raised out to the side. Understanding the anatomy and biomechanics of the infraspinatus is especially relevant in fields such as sports medicine, physical therapy, and orthopedics, where shoulder injuries and rotator cuff issues are often addressed.