The Lying Triceps Extension, widely known as the “Skull Crusher,” is a highly effective isolation movement for the muscles on the back of the upper arm. Performed while lying on a flat surface, a weight is lowered toward the head by bending only at the elbow joint. The exercise earns its dramatic nickname because the movement path brings the weight close to the forehead or crown. It is a staple in many strength programs, prized for its ability to target and develop the triceps muscle group.
Primary Muscle Activation
The primary muscle worked by the skull crusher is the Triceps Brachii, a large, three-headed muscle that makes up roughly two-thirds of the upper arm mass. The name “triceps” refers to its three distinct origins: the long head, the lateral head, and the medial head. All three heads converge into a single tendon that inserts onto the ulna, making their collective action the extension, or straightening, of the elbow joint.
The arm positioning during the skull crusher places unique emphasis on the long head. Unlike the medial and lateral heads, which originate on the humerus, the long head originates on the scapula. Because it crosses the shoulder joint, the long head is placed under a significant stretch when the arm is positioned overhead, as it is in the starting position of the skull crusher.
This pre-stretch makes the long head a major contributor to the movement, maximizing its activation and growth potential. While the medial and lateral heads are also heavily engaged in elbow extension, the long head is specifically challenged by the overhead nature of the lift. The medial head is noted for its consistent activation throughout the entire range of motion of triceps exercises.
Secondary muscles involved are mainly stabilizers that keep the body and upper arm in the correct position. The anterior deltoids and pectorals assist in maintaining the fixed overhead position of the upper arm. Forearm muscles also work isometrically to stabilize the wrist and grip the weight.
Execution and Form
To perform the skull crusher safely, lie supine on a flat bench with your feet planted firmly on the floor. Grasp an EZ-curl bar or straight barbell with a shoulder-width, pronated grip, extending the weight above your chest or slightly behind your head. The upper arms should be nearly perpendicular to the floor, or slightly angled back, and must remain stationary throughout the exercise.
Initiate the movement by slowly bending only at the elbows, lowering the weight in a controlled manner toward the crown of your head or just beyond it. The elbows should point forward and not flare out to the sides, as this shifts tension away from the triceps and can strain the elbow joints. Lower the weight until the forearms are approximately parallel to the floor, achieving a deep triceps stretch.
Reverse the motion by powerfully extending the elbows, squeezing the triceps to press the weight back to the starting position. Avoid fully locking out the elbows at the top of the movement to maintain continuous tension on the muscle. Using too much weight is a common error, which can cause the upper arms to drift forward or the lower back to arch, reducing triceps isolation and increasing injury risk.
Variations and Alternatives
Several variations of the skull crusher exist to accommodate different equipment and training goals. Using dumbbells instead of a barbell is a popular modification that allows for a more natural, neutral grip, which can reduce wrist and elbow discomfort. Dumbbells also permit unilateral training, helping to correct strength imbalances between the arms.
Changing the surface angle is another way to modify the exercise, such as performing the movement on an incline bench. This variation increases the stretch and activation on the long head of the triceps due to the more acute overhead angle of the shoulder joint. Conversely, performing the movement on a decline bench slightly reduces the involvement of the long head.
For those seeking constant tension, the cable machine offers a distinct alternative. Cable overhead triceps extensions eliminate the “sticking point” often felt with free weights, providing uniform resistance throughout the repetition. Bodyweight alternatives, like the close-grip or diamond push-up, also serve as effective substitutions by heavily recruiting the triceps for elbow extension.