The underhand pull-up, commonly referred to as the chin-up, is a foundational bodyweight exercise for developing upper body strength. This movement involves grasping a horizontal bar with a supinated grip, meaning the palms face toward the body, and pulling the torso upward until the chin clears the bar. The specific orientation of the hands fundamentally alters the biomechanics of the exercise compared to an overhand pull-up.
How the Underhand Grip Changes Muscle Activation
The supinated, or underhand, grip places the forearm in a position that gives a mechanical advantage to the primary elbow flexors. This grip orientation allows the biceps brachii muscle to generate a stronger pull compared to the overhand grip, where the biceps are less efficiently recruited. Studies have consistently shown a higher level of biceps activation during the chin-up compared to the pronated-grip pull-up. This change in hand position shifts a portion of the workload away from the back musculature and onto the muscles of the anterior arm.
The brachialis, a powerful elbow flexor that lies beneath the biceps, is also highly active in this position, contributing significantly to the pulling force. The brachioradialis, an elbow flexor in the forearm, also assists, though its activation tends to be lower than the biceps.
Primary Muscles for Pulling and Arm Flexion
The two main muscle groups driving the upward phase of the underhand pull-up are the Latissimus Dorsi and the Biceps Brachii. The Latissimus Dorsi, the broadest muscle of the back, functions as the primary mover for the entire vertical pull. Its action involves shoulder extension and adduction, bringing the upper arms down and back toward the torso from an overhead position. During the upward phase, the Lats shorten to execute the powerful pulling motion.
The Biceps Brachii acts as the chief synergist for the Lats, primarily responsible for flexing the elbow joint. Due to the underhand grip, the biceps are placed at an optimal length-tension relationship, maximizing their contribution to lifting the body against gravity. The peak activation of the biceps occurs during the upward portion of the lift, as the elbow angle decreases to bring the chin to the bar. These two muscle groups work in concert, with the Lats providing the large back movement and the Biceps ensuring the arms bend effectively to complete the range of motion.
Supporting Back and Stabilizing Muscles
Beyond the primary movers, several smaller muscles act as synergists and stabilizers to ensure an efficient movement pattern. The Teres Major works alongside the Latissimus Dorsi to assist with shoulder extension and internal rotation. The Rhomboids and the Middle Trapezius muscles are recruited to retract and depress the shoulder blades, creating a stable base for the arms to pull from.
The Posterior Deltoids contribute to the shoulder extension component of the movement, helping the Lats pull the arms down. Furthermore, the underhand pull-up requires significant engagement from the deep core musculature. The Rectus Abdominis and Obliques contract isometrically throughout the movement to maintain a straight, rigid body line, ensuring the force generated is translated efficiently into vertical motion.