Final answer:
The subscapularis muscle is the most powerful of the rotator cuff muscles and is responsible for medially rotating the arm. Its tendons alongside those of the supraspinatus, infraspinatus, and teres minor, form the musculotendinous cuff which provides support to the shoulder joint.
Step-by-step explanation:
The subscapularis muscle is one of the four rotator cuff muscles that provide support to the shoulder joint. This muscle is noted for being the most powerful muscle of the rotator cuff group. It attaches to the humerus and originates on the anterior side of the scapula, playing a key role in medially rotating the arm. These muscles, which also include the supraspinatus, infraspinatus, and teres minor, are connected to the humerus via tendons that form a strong connective tissue structure known as the musculotendinous cuff. This tissue surrounds and supports the superior, anterior, lateral, and posterior sides of the humeral head. As dynamic ligaments, they adjust their contraction strength to provide primary structural support to the glenohumeral joint, effectively holding the head of the humerus in the glenoid cavity.