Final answer:
The Infraspinatus muscle, part of the rotator cuff, is located under the scapula's spine, enabling arm lateral rotation and contributing to shoulder stability.
Step-by-step explanation:
The Infraspinatus muscle is one of the four muscles that make up the rotator cuff, a group of muscles and tendons that provide stability and movement to the shoulder joint. This particular muscle is located inferior to the spine of the scapula and is responsible for the lateral rotation of the arm. The rotator cuff muscles, including the infraspinatus, supraspinatus, teres minor, and subscapularis, arise from the scapula and attach to the humerus, encircling the head of the humerus and fusing with the shoulder joint capsule to offer support and facilitate various arm movements.
The infraspinatus, along with the other rotator cuff muscles, also helps to hold the head of the humerus within the glenoid cavity, effectively acting as 'dynamic ligaments' to promote shoulder stability.