Which muscle forms the curve of the shoulder?

The deltoid muscle forms the curve of the shoulder due to its anatomical position and unique structure. This muscle is primarily responsible for the rounded contour of the shoulder and plays a significant role in arm movements, particularly in abduction, flexion, and extension of the shoulder joint.

The deltoid has three distinct parts: the anterior, lateral, and posterior fibers, each contributing to the muscle's ability to provide a wide range of motion while also allowing for stability at the shoulder. Its location on the outer shoulder makes it the most visible muscle in creating that characteristic shoulder shape, distinguishing it from the other muscles listed, which serve different functions in the shoulder and surrounding areas.

For instance, the trapezius muscle primarily facilitates movements of the scapula and upper back, the pectoralis major mainly contributes to movements of the arm and is located more anteriorly in the chest, and the biceps brachii is largely involved in flexing the elbow and supination of the forearm. None of these muscles create the signature curve of the shoulder to the same extent as the deltoid.