Which muscle is involved in adduction and medial rotation of the shoulder joint?

The pectoralis major is primarily responsible for the actions of adduction and medial rotation of the shoulder joint. This large muscle is situated at the front of the chest and is crucial for movements that bring the arm closer to the body (adduction) and rotate the arm inward (medial rotation).

When the pectoralis major contracts, it pulls the humerus towards the body's midline, effectively achieving adduction. Additionally, due to its anatomical positioning, when the arm is raised, this muscle also assists in medially rotating the shoulder.

The deltoids are mainly responsible for arm abduction and can assist in some degree of rotation, but their primary function is not adduction or medial rotation. The coracobrachialis plays a role in flexion and adduction as well, but it is not the primary muscle for medial rotation. The superior trapezius is mainly involved in elevating and rotating the scapula rather than directly influencing shoulder adduction or rotation.

Therefore, the pectoralis major is the key muscle involved in both adduction and medial rotation of the shoulder joint, making it the correct answer.