From the Research
When the elbow is completely flexed, the biceps brachii muscle abducts forward, but the primary muscle responsible for this action is not explicitly stated in the provided evidence, however, based on the anatomy and function of the muscles around the elbow, the brachialis muscle is the most likely candidate to abduct forward in this position. The brachialis muscle is a key flexor muscle of the elbow joint, located deep to the biceps brachii on the anterior surface of the humerus. In a position of complete elbow flexion, the brachialis continues to exert its force, pulling the forearm forward and upward. This muscle originates on the lower half of the anterior humerus and inserts on the ulnar tuberosity, giving it mechanical advantage for elbow flexion regardless of forearm position. Unlike the biceps brachii, which is also an elbow flexor but functions optimally when the forearm is supinated, the brachialis works consistently in all forearm positions 1, 2. The brachialis is sometimes called the "workhorse" of elbow flexion because it contributes significantly to flexion strength and is active during all elbow flexion movements, including the final degrees of flexion when the elbow is nearly completely bent. Some studies have investigated the role of the elbow musculature in elbow stability, and have found that the medial elbow musculature, which includes the brachialis, plays a significant role in providing stability to the elbow, especially when the forearm is supinated 3. Other studies have examined the effect of arm position on the EMG reaction time of the biceps brachii for elbow flexion and forearm supination, and have found that the reaction time is influenced by the starting position of the arm and the presence or absence of a warning signal 4, 5. However, these studies do not provide direct evidence for the muscle that abducts forward when the elbow is completely flexed. Therefore, based on the anatomy and function of the muscles around the elbow, the brachialis muscle is the most likely candidate to abduct forward when the elbow is completely flexed.