Which muscle covers the shoulder joint completely?

The deltoid muscle is the one that completely covers the shoulder joint. This muscle is shaped like an inverted triangle and is situated on the outer aspect of the shoulder. It plays a vital role in the movement and stabilization of the shoulder and provides the primary contour of the shoulder's shape.

The deltoid is responsible for several actions, including abduction, flexion, and extension of the arm, making it essential for various overhead movements. Its anatomical structure allows it to encapsulate the shoulder joint, creating a protective layer that contributes to the joint's functionality and mobility.

In contrast, other muscles mentioned do not serve this specific purpose. The sartorius is a long, thin muscle that runs down the length of the thigh and is not related to shoulder movement. The trapezius, while a large muscle that extends from the back of the neck and upper spine down to the middle of the back, primarily supports movements of the neck, shoulder girdle, and upper limbs rather than enclosing the shoulder joint entirely. The quadriceps is a group of muscles located in the front of the thigh and is primarily involved in knee extension. None of these other muscles have the same role as the deltoid in covering and functioning around the shoulder joint