What is the large, thick triangular muscle that covers the shoulder and lifts the arm?

The deltoid muscle is a large, thick triangular muscle that is particularly significant for its role in shoulder movement. This muscle is located on the outer aspect of the shoulder and is responsible for lifting the arm, enabling actions such as reaching, pushing, and lifting. Its shape resembles a triangle, which contributes to the fullness of the shoulder's contour.

The deltoid has three distinct parts or “heads” – the anterior (front), lateral (side), and posterior (rear) – each contributing to different movements of the arm. When the muscle contracts, it facilitates abduction of the arm, especially when the arm is lifted away from the body, making it essential for many upper body activities.

In comparison, the pectoralis major is primarily involved in movements like adduction and rotation of the shoulder but does not cover the shoulder in the same way. The trapezius, while it's important for shoulder stability and moving the shoulder blades, does not have the same direct function in lifting the arm. The biceps brachii is primarily a flexor of the elbow and does not significantly contribute to shoulder lifting. This context helps to clarify why the deltoid is accurately identified as the muscle responsible for covering the shoulder and lifting the