Final answer:
The muscle in question that is innervated by the radial nerve and assists with forearm flexion, sharing a common action with the biceps brachii and brachioradialis, is the brachioradialis muscle.
Step-by-step explanation:
The muscle described as being served by the radial nerve, with an origin on the lateral epicondyle of the humerus and the ulna and an insertion on the radius that shares an action with the biceps brachii and the brachioradialis is the brachioradialis muscle.This muscle can flex the forearm quickly or help lift a load slowly. The action it has in common with the biceps brachii and the brachioradialis muscles is the flexion of the forearm. Since the brachialis is deep to the biceps brachii and provides additional power in flexing the forearm, it is closely related in function but not served by the radial nerve.This muscle has one action in common with the biceps brachii and the brachialis muscles, which is flexing the forearm. The brachioradialis can quickly flex the forearm or help lift a load slowly.