Final answer:
The triceps brachii is the muscle responsible for extending the forearm. It is an extensor muscle, opposing the biceps brachii, which is a flexor muscle of the arm.
Step-by-step explanation:
The large muscle of the arm that is responsible for extending the forearm is known as the triceps brachii. This three-headed muscle contrasts with the flexor muscle, the biceps brachii, which is responsible for closing the limb, or flexing the elbow. The triceps brachii functions as an extensor that opens the limb, allowing for the straightening of the elbow joint. Furthermore, skeletal muscles such as the triceps and biceps are arranged in pairs to facilitate the movement and mechanical action of the limbs. The tendons of these muscles attach to bones near the joints, which amplifies the force they can exert within the body compared to the force applied externally.