The skull crusher, or lying triceps extension, is a popular exercise designed to build mass and strength in the back of the upper arm. The muscle group targeted is the triceps brachii, which is the primary muscle responsible for straightening the elbow joint. Many people perform this movement to specifically target the long head of the triceps for maximum arm development.
The Three Heads of the Triceps Muscle
The triceps brachii muscle is composed of three distinct sections: the medial, lateral, and long heads. All three heads converge into a single tendon that crosses the elbow joint and attaches to the ulna, making them all elbow extensors. The medial and lateral heads originate on the humerus. These two heads function almost exclusively to extend the forearm at the elbow joint.
The long head possesses a unique anatomical distinction that changes its function. It originates from the infraglenoid tubercle of the scapula, meaning it crosses both the elbow and the shoulder joints. Because of this attachment, the long head is considered a bi-articular muscle, assisting in the extension and adduction of the arm at the shoulder joint. This dual attachment point means the long head’s activation and length are significantly influenced by the position of the upper arm relative to the torso.
Biomechanical Analysis of the Skull Crusher Movement
The skull crusher exercise is performed with the upper arm positioned in shoulder flexion, meaning the arms are raised and held overhead. Placing the arm in this overhead position stretches the long head of the triceps across the shoulder joint. This pre-stretching places the long head at a mechanically advantageous length for force production during the subsequent elbow extension.
When a muscle is stretched to a longer length, it has a greater potential for activation and growth, a phenomenon known as the length-tension relationship. Conversely, the medial and lateral heads, which only cross the elbow, are not affected by the shoulder position in the same way. The long head is therefore placed under a high degree of tension at the bottom of the skull crusher movement.
The movement requires the lifter to bend the elbows, lowering the weight toward the head while keeping the upper arms relatively stable. The intense stretch felt at the bottom range of motion is a direct indication of the long head being heavily loaded. The exercise design forces the long head to work through a large range of motion while under significant stretch, which is a potent stimulus for muscle hypertrophy.
Maximizing Long Head Tension During Execution
To ensure the skull crusher maximally targets the long head, slight adjustments to the standard technique can be beneficial. Instead of keeping the upper arms perfectly perpendicular to the floor, angling them back slightly increases the shoulder flexion. This small shift increases the stretch placed upon the long head at the start of the movement, which maintains a greater level of tension throughout the set.
Controlling the tempo, particularly the eccentric (lowering) phase of the lift, further enhances the muscle-building stimulus on the long head. The muscle is under the most stretch and tension during the bottom portion of the movement, so a slow and controlled descent is advantageous.
It is also beneficial to stop the lift just short of full elbow lockout at the top of the repetition. Avoiding the full lockout maintains constant tension on the triceps, preventing the long head from entering a momentary state of rest. Many lifters find that using an EZ-curl bar instead of a straight barbell improves wrist comfort. Maintaining a stable torso and keeping the elbows pointed forward ensures the triceps remain isolated.