When asked to flex a muscle, you'll most likely flex your biceps by bending your elbow. Although the biceps muscle is involved with flexion of the elbow, contrary to popular belief, it's not always the most active elbow flexor. Several other arm muscles are involved in elbow flexion and can be more active than the biceps, depending on the position of your arm.
The biceps brachii, a two-headed muscle, sits on the front of your upper arm and is the most visible elbow flexor. It originates on the scapula, runs the length of your upper arm and attaches to the bones of the forearm, the ulna and radius. The short head of the biceps brachii sits on the pinky side of the upper arm and the long head sits on the thumb side. Besides elbow flexion, the biceps muscle is also involved in forearm supination -- turning your forearm so your palm faces up. It's more active in elbow flexion when your forearm is supinated and less active when your forearm is pronated -- your palm faces down. The short head of the biceps brachii is involved in flexing the shoulder, or lifting your upper arm in front of your body. This means the short head is not as active in flexing the elbow when your shoulder is flexed.
The brachioradialis is a long, thin muscle that extends down the thumb side of your forearm. It originates on your humerus, or upper-arm bone, crosses the elbow joint and attaches on the lower part of the radius, the smaller of the two forearm bones. The brachioradialis is more active in flexing the elbow when your forearm is pronated, which is when the biceps brachii is at a disadvantage.
The brachialis muscle lies underneath the biceps brachii. It originates on the lower half of the humerus bone, crosses the elbow joint and attaches to the ulna, the larger of the two forearm bones. The brachialis muscle is not involved in elbow pronation or supination and is active in elbow flexion regardless of the angle of the forearm.
The pronator teres, flexor carpi radialis and extensor carpi radialis longus, three small muscles in the forearm, are also involved in elbow flexion, but not to the extent of the biceps, brachialis and brachioradialis. These three small forearm muscles are mainly responsible for movements of the wrist and forearm, but they do assist during elbow flexion.