What type of muscle is primarily responsible for extension in the arm?

The triceps brachii is primarily responsible for extension in the arm, making it the correct choice for this question. The triceps is a muscle located at the back of the upper arm and is composed of three heads (long, lateral, and medial), allowing it to effectively extend the elbow joint when contracted. When you perform actions like pushing or straightening your arm, the triceps is the main muscle facilitating this movement, counteracting the flexor muscles such as the biceps brachii.

In contrast, the biceps brachii, which is primarily involved in flexion at the elbow, works to bring the forearm closer to the upper arm. The deltoids, while important for various movements of the shoulder, do not primarily function in elbow extension. Lastly, the teres major assists in shoulder extension and internal rotation but does not play a significant role in arm extension at the elbow joint. Thus, the triceps brachii’s role in arm extension is clearly defined, making it the correct answer in this context.