What is the name of the large, flat, triangular muscle covering the lower back that helps extend the arm?

The large, flat, triangular muscle that covers the lower back and assists in extending the arm is known as the latissimus dorsi. This muscle plays a crucial role in various arm movements, including adduction, extension, and internal rotation. It originates from the lower spine and iliac crest and inserts into the upper arm bone, allowing it to have a significant impact on shoulder mobility and strength. The latissimus dorsi is also involved in actions such as pulling and lifting, making it essential for activities that require upper body strength.

In contrast, the trapezius is located in the upper back and is primarily responsible for moving and stabilizing the shoulder blades. The deltoid is the muscle that gives the shoulder its rounded contour and is mainly involved in raising the arm, while the rhomboid muscles are responsible for retracting the shoulder blades. Each of these muscles has distinct roles, but the latissimus dorsi is specifically noted for its function in extending the arm and its broad coverage over the lower back area.