What is one of the primary functions of the supraspinatus muscle?

The supraspinatus muscle plays a pivotal role in the abduction of the humerus, particularly during the initial phase of arm raising, and it also contributes to the stabilization of the head of the humerus within the glenoid cavity of the scapula.

When the arm is raised, the supraspinatus activates to assist in lifting the arm away from the body, effectively facilitating the abduction movement. This muscle helps maintain proper alignment and centration of the humeral head during arm movements, preventing dislocation and ensuring smooth motion. As part of the rotator cuff group, the supraspinatus provides essential support to the shoulder joint by keeping the humeral head positioned properly against the glenoid fossa, thereby enhancing overall shoulder stability and function.

The other options refer to different muscle functions or anatomical actions not primarily associated with the supraspinatus. For instance, medial rotation typically involves the subscapularis muscle, while extension and downward rotation of the scapula is primarily the action of the trapezius and rhomboid muscles. The reinforcement of the shoulder joint capsule is a function attributed to the overall rotator cuff, but not specifically to the supraspinatus muscle itself.