Final answer:
The deltoid muscle is the powerful abductor of the upper arm, while the triceps brachii muscle is responsible for extending the forearm.
Step-by-step explanation:
The powerful abductor muscle of the upper arm is the deltoid muscle. This muscle is not only responsible for abducting the arm but also plays a role in flexing and medial rotation, as well as extension and lateral rotation. Regarding the extension of the forearm, this movement is primarily performed by the triceps brachii, which is located on the posterior side of the upper arm and serves as the antagonist to the biceps brachii during forearm flexion.