The preacher curl is a resistance exercise that intensely focuses on the musculature of the upper arm. It is performed by bracing the upper arms against a specialized, angled pad, which locks the body into a fixed position. Because the torso and shoulders cannot assist in the lift, the exercise is highly effective for isolating the elbow flexors. This restriction forces the targeted muscles to bear the entire load, making it a powerful tool for developing specific parts of the biceps.
The Anatomy of the Biceps Brachii
The muscle commonly known as the biceps is anatomically called the biceps brachii, meaning “two-headed muscle of the arm.” It is composed of two distinct sections that originate at the shoulder but merge before inserting into the forearm. The short head is the medial portion, originating from the coracoid process of the scapula.
The long head is the lateral portion, originating from the supraglenoid tubercle of the scapula. Both heads converge into a single tendon that attaches to the radial tuberosity in the forearm. This dual structure allows the biceps brachii to perform two primary actions: elbow flexion and supination (turning the palm upward).
Mechanical Isolation: The Role of the Preacher Bench
The unique effectiveness of the preacher curl stems from the mechanical function of the preacher bench apparatus. The pad supports and fixes the upper arm in a forward, slightly flexed position at the shoulder joint. This fixation prevents the body from using momentum or recruiting larger muscle groups, such as the anterior deltoids, to assist the lift.
By eliminating the ability to swing the weight, the bench enforces a strict range of motion. The biceps must initiate and control the movement through the entire repetition. This strict control maximizes the tension placed directly onto the elbow flexors, ensuring the work is a result of pure muscular contraction.
Primary Target: Emphasis on the Short Head
The preacher curl’s specific arm position creates a biomechanical disadvantage for the long head, shifting the workload to the short head. Placing the upper arm on the angled pad puts the shoulder joint into flexion, passively shortening the long head of the biceps. A muscle shortened at one joint has reduced leverage and less capacity to generate force at the elbow joint.
This mechanical limitation minimizes the recruitment of the long head, forcing the medial short head to take on the majority of the load. The short head is highly active during elbow flexion, especially in the contracted position. Training the short head contributes to the overall width and thickness of the biceps muscle when viewed from the front.
Secondary Muscles and Grip Influence
While the biceps brachii is the primary mover, the preacher curl also heavily engages two secondary muscles. The brachialis lies directly beneath the biceps brachii and is a powerful, pure elbow flexor that does not contribute to forearm supination. As the brachialis grows, it pushes the biceps upward, contributing significantly to the overall size and thickness of the upper arm.
The brachioradialis, a muscle of the forearm, also assists with elbow flexion. During the preacher curl, the brachioradialis helps stabilize the wrist and control the weight, especially during the lowering phase. Minor adjustments to grip width can subtly alter the focus, but these changes are secondary to the isolation effect created by the fixed angle of the bench.