The deltoid muscle is divided into three heads: the anterior (front), the posterior (back), and the lateral (side) deltoid. Each head is responsible for unique movements of the arm. Understanding how different strength training exercises engage these separate muscle heads is necessary for balanced upper body development. The bench press is a foundational upper body movement, but the degree to which it stimulates the lateral deltoid is often misunderstood.
Primary Muscle Activation During Bench Press
The bench press is a compound exercise involving multiple joints and muscle groups to push weight away from the chest. The primary muscle generating the force is the Pectoralis Major, which is responsible for the horizontal adduction of the humerus, or bringing the upper arm toward the midline of the body. The greatest electrical activity in the pectoralis major occurs during the initial ascent phase of the lift.
Two other muscle groups act as synergists to complete the pressing motion. The Triceps Brachii extends the elbow, which is necessary to straighten the arms and lock out the weight at the top of the repetition. Studies often show the pectoralis major, followed by the anterior deltoid and the triceps brachii, as the most activated muscles during the bench press.
The Anterior Deltoid plays a significant role by assisting the chest in horizontal adduction and shoulder flexion. As the bar is lowered in the eccentric phase, the anterior deltoid contracts to control the movement, and it continues to contract powerfully during the concentric, or pushing, phase. Incline bench press variations, particularly those with a steeper angle, place greater emphasis on the activation of the anterior deltoid. The mechanical demands of the bench press align perfectly with the function of the anterior head, ensuring it receives substantial training stimulus.
The Role of the Side Deltoid
The lateral deltoid head has a different primary function from its anterior counterpart. Its main role is shoulder abduction, which is the movement of lifting the arm laterally away from the side of the body. This movement occurs primarily in the coronal plane, which is perpendicular to the horizontal pushing motion of the bench press. Since the bench press involves horizontal adduction and elbow extension, the lateral deltoid is not a primary mover of the weight.
Because the path of the bar is largely horizontal, the lateral deltoid does not undergo a significant change in length under tension, which is necessary for maximizing muscle growth. While the lateral deltoid may contribute to stabilizing the shoulder joint during the push, this role results in minimal mechanical tension compared to the targeted muscles. The stabilizing function is insufficient to produce the high levels of muscular activation needed for substantial hypertrophy.
Research indicates that the lateral deltoid shows little electrical activity during the bench press compared to exercises specifically designed for it. The movement pattern of the bench press does not involve the shoulder abduction necessary to challenge the lateral head effectively. Therefore, using the bench press as a standalone exercise for comprehensive shoulder development will likely result in an imbalance between the front and side deltoids.
Exercises for Comprehensive Shoulder Development
Since the bench press trains the anterior deltoid and not the lateral head, specific exercises are necessary to target the side of the shoulder effectively. These exercises must incorporate the primary function of the lateral deltoid: shoulder abduction. The dumbbell lateral raise is widely considered the gold standard for isolating this muscle head.
The movement involves lifting dumbbells out to the sides until the arms are parallel to the floor, directly engaging the lateral deltoid. To maintain constant tension, cable lateral raises can be used, which may enhance the training stimulus. Cable machines provide consistent resistance, unlike free weights where tension can drop off at certain points in the range of motion.
Other variations, such as the incline bench lateral raise, minimize momentum and isolate the lateral deltoid by locking the torso in place. Upright rows, when performed with a wide grip and stopping the pull at shoulder height, also place greater work on the side deltoids. Incorporating these abduction-focused movements ensures the lateral deltoid receives adequate mechanical tension to promote balanced shoulder growth.