Identify the muscle based on its actions: extension, adduction, and medial rotation at the shoulder.

The muscle responsible for extension, adduction, and medial rotation at the shoulder is the latissimus dorsi. This broad, flat muscle covers a large area of the lower back and is primarily involved in movements that draw the arm down and backwards, such as in swimming or pulling actions.

The reason the latissimus dorsi is identified with these actions is due to its anatomical position and attachment points. It originates from the lower thoracic and lumbar vertebrae, as well as the iliac crest and lower ribs. Its fibers converge into a tendon that inserts into the intertubercular groove of the humerus, allowing it to influence shoulder joint movements effectively.

As for the other muscles: while the deltoid can assist with shoulder movements, its primary functions are abduction and flexion. The subscapularis mainly contributes to medial rotation but does not significantly perform extension or adduction. The pectoralis major is primarily involved in flexion and adduction of the shoulder, but it does not have a strong role in extension when the arm is already extended, particularly from the anatomical position.

In summary, the latissimus dorsi is correctly identified for its synergistic role in performing extension, adduction, and