Which muscle is primarily responsible for extending the elbow and shoulder?

The triceps brachii is the primary muscle responsible for extending the elbow and shoulder due to its anatomical structure and muscle fiber orientation. The triceps is situated at the back of the upper arm and consists of three heads: the long head, lateral head, and medial head. The long head of the triceps originates from the scapula, allowing it to play a role in shoulder extension in addition to elbow extension, which is primarily executed by all three heads.

The muscle's function in elbow extension involves straightening the arm, making it crucial during many pushing movements or activities that require arm extension. Similarly, when the arm is raised overhead or moved back, the long head contributes to shoulder extension, working in coordination with other muscles such as the deltoids and latissimus dorsi. This dual functionality of the triceps brachii for both joints distinguishes it as the primary mover in these actions.