Final answer:
The brachialis muscle inserts on the ulna and is involved in powering forearm flexion.
Step-by-step explanation:
The insertion for the brachialis muscle is located on the ulna, which is one of the bones of the forearm. The brachialis muscle is situated deep to the biceps brachii, and it is a key muscle that provides power when flexing the forearm. This muscle works in conjunction with the biceps brachii and the brachioradialis muscle, where the latter can also flex the forearm quickly or help lift a load slowly.