The Biceps Brachii is the primary muscle group responsible for bending the elbow, a movement commonly known as a curl. This muscle is unique because it is composed of two distinct parts, referred to as “heads,” which attach at different points on the shoulder blade. Since both the long head and the short head contribute to elbow flexion, it is impossible to completely isolate one from the other during a curl. However, by changing the arm’s position relative to the torso, it is possible to significantly shift the mechanical emphasis, allowing exercisers to strategically target one head over the other through specific curl variations.
The Anatomy of Biceps Heads
The two heads of the Biceps Brachii are distinguished by their points of origin on the scapula, which dictates their function at both the shoulder and elbow joints. The long head is situated on the outer side of the arm and originates from the supraglenoid tubercle, an attachment point located just above the socket of the shoulder joint. Because its tendon travels through the shoulder joint, the long head is considered a bi-articular muscle, meaning it acts across two joints—the shoulder and the elbow.
The short head, found on the inner side of the arm, originates from the coracoid process of the scapula. This attachment point is located away from the main shoulder joint capsule, making the short head’s role at the shoulder less complex than that of the long head. Both heads converge to insert distally onto the radial tuberosity of the forearm, sharing the common action of flexing the elbow and powerfully supinating the forearm.
The bi-articular nature of the long head means that any change in shoulder position, such as moving the arm backward into extension, will place a greater stretch on this head. This pre-stretching increases the long head’s potential for activation and force production when the subsequent elbow flexion occurs. The short head, however, is mechanically favored when the shoulder is placed in a slightly flexed position, such as when the arm is positioned in front of the body.
Exercise Variations That Target the Long Head
To emphasize the long head, the goal is to place the muscle in a maximally stretched position at the start of the curl. This is achieved by moving the arm behind the line of the torso, which puts the shoulder joint into extension. The increased stretch on the long head at this angle allows it to contribute a greater proportion of the total force during the elbow flexion movement.
The Incline Dumbbell Curl is the most direct application of this principle, as the exerciser leans back on an incline bench, allowing the arms to hang down and placing the shoulder in a degree of extension. This stretched starting position maximizes activation for the long head throughout the entire range of motion of the curl. This exercise is particularly effective at improving the force-producing capacity of the biceps.
Another effective variation is the Hammer Curl, which uses a neutral grip where the palms face each other. This hand position significantly reduces the biceps’ ability to supinate the forearm, which is a primary action of the Biceps Brachii. Since the long head is strongly recruited in this neutral position, the hammer curl increases its relative activation compared to a traditional supinated curl.
Exercise Variations That Target the Short Head
Emphasizing the short head requires placing the arm in a position of shoulder flexion, which means the arm is held forward and supported against a surface. This position shortens the long head at the shoulder joint, effectively minimizing its involvement and shifting the workload primarily to the short head for elbow flexion. This mechanical advantage is often used to develop the thickness and roundness of the biceps muscle belly.
The Preacher Curl is the most common exercise that achieves this by having the exerciser rest the upper arm against a padded support. Since the arm is pinned in front of the body and the shoulder is flexed, the short head is forced to carry the majority of the load. Studies show that this exercise elicits a high level of muscle activation, particularly in the beginning portion of the curl’s upward phase.
Using a Wide-Grip Barbell Curl also helps to shift the emphasis toward the short head, especially when the grip is fully supinated (palms up). A wider grip naturally encourages a greater degree of forearm supination, which is an action the short head assists with strongly. This altered line of pull and the requirement for maximal supination combine to favor the recruitment of the head of the muscle.
The Role of Auxiliary Arm Muscles in Curls
While the Biceps Brachii receives the most attention, two other muscles play a substantial role in elbow flexion and arm development: the Brachialis and the Brachioradialis.
The Brachialis
The Brachialis is situated underneath the biceps and acts as the strongest pure flexor of the elbow joint. Unlike the biceps, the Brachialis only crosses the elbow joint and does not contribute to forearm supination, meaning it is active regardless of the hand position. It is heavily recruited during all curling movements, especially those performed with a neutral grip, such as the Hammer Curl. Developing the Brachialis is beneficial because its growth pushes the Biceps Brachii upward, which can enhance the appearance of the biceps peak.
The Brachioradialis
The Brachioradialis is a prominent forearm muscle that also acts as an elbow flexor, particularly when the forearm is held in a neutral or pronated (palm down) position. It is notably active during Hammer Curls, alongside the long head of the biceps and the Brachialis, due to the neutral hand position. These auxiliary muscles ensure that arm development is comprehensive. Focusing solely on the Biceps Brachii neglects the substantial mass and strength contribution of the underlying musculature. Therefore, a complete arm training program incorporates variations that effectively target all three elbow flexors.