Final answer:
The statement is false; the biceps brachii, not the brachioradialis, is the primary elbow flexor when the forearm is fully supinated as the biceps brachii also assists in supination.
Step-by-step explanation:
The brachioradialis is a muscle that plays a role in flexing the forearm, especially when the forearm is in the mid-position between pronation and supination. When it comes to fully supinating the forearm, the main muscle responsible for this action is the biceps brachii.
The biceps brachii crosses both the shoulder and elbow joints and is active in forearm flexion and also in turning the forearm anteriorly, which is known as supination. Therefore, the statement that the 'brachioradialis is the primary elbow flexor when the forearm is fully supinated' is false. The primary elbow flexor when the forearm is fully supinated is actually the biceps brachii, due to its role in forearm flexion and supination at the radioulnar joints.