The barbell curl is a fundamental exercise for building upper arm strength and size, primarily targeting the biceps brachii. This movement also engages other muscles that contribute significantly to the overall thickness and function of the arm. Understanding the distinct roles of these muscles clarifies exactly what part of the arm is being worked during a standard barbell curl.
Understanding the Anatomy of Arm Flexion
The action of bending the elbow, known as elbow flexion, involves three primary muscles working in synergy. The biceps brachii is the most visible, consisting of two separate heads that merge into a single muscle belly. Deeper underneath the biceps is the brachialis, a powerful and dedicated elbow flexor. The brachialis originates on the humerus and inserts on the ulna, meaning its function is unaffected by forearm rotation.
The third muscle is the brachioradialis, a forearm muscle that also contributes to elbow flexion. It runs along the thumb side of the forearm and is a strong flexor when the forearm is in a neutral position (palm perpendicular to the ground). Because the brachialis is active regardless of forearm position, it is considered the “pure flexor” of the elbow. Developing the brachialis can increase the width of the arm and enhance the appearance of greater size.
Primary Activation: The Short and Long Heads
The biceps brachii muscle is composed of two distinct parts: the long head and the short head. The long head is situated on the outer side of the arm and is responsible for the height or “peak” of the muscle. Its tendon passes through the shoulder joint, meaning its activation is influenced by shoulder position.
The short head is located on the inner side of the arm and contributes to the overall width and fullness of the biceps. A traditional barbell curl uses a supinated (palms-up) grip, which maximizes the activation of the biceps brachii because the muscle is a strong forearm supinator. When performing a standard curl with a shoulder-width grip, both heads are significantly engaged, though the short head is often slightly more emphasized.
How Grip Adjustments Shift Muscle Focus
Varying the grip width is the primary way to subtly shift focus between the two heads of the biceps brachii. A grip significantly wider than shoulder-width places greater emphasis on the short head, or the inner portion of the biceps. This wider position slightly rotates the shoulders, directing more tension toward the inner muscle.
Conversely, a narrower grip (hands closer than shoulder-width) places greater emphasis on the long head, or the outer portion of the biceps. A narrow grip biases the long head and is often used to develop the muscle’s peak. Shifting away from the supinated grip to a neutral or pronated grip significantly reduces biceps brachii activation, instead maximizing engagement of the brachialis and brachioradialis.