Summary
Jeff Cavaliere of ATHLEANX explains the most common mechanical error in the Skullcrusher exercise that eliminates triceps activation at the top of the movement. By applying basic physics principles, he demonstrates how arm positioning relative to gravity determines whether the muscle is actually working. A simple repositioning of the starting arm angle fixes the problem entirely.
Key Points
- Most people perform Skullcrushers incorrectly by positioning the arm straight up, perpendicular to the floor, in the fully contracted position
- The “Skullcrusher” name itself causes the mistake — people aim the bar toward their skull or nose, which leads to the wrong arm angle
- At full extension, the forearm should NOT be vertical — when the forearm is parallel to the force of gravity (straight up), no muscular work is being done by the triceps
- The lever arm is the forearm — for the triceps to produce force, the forearm must be at an angle to gravity, not aligned with it
- The correct setup angles the arms back slightly so that even in the fully contracted (extended) position, there is still an angle between the forearm and the downward pull of gravity
- The arc of the bar travels back behind the head rather than straight down toward the face, meaning the bar never approaches the skull or nose
- This principle applies whether using a barbell, EZ curl bar, or dumbbells
Exercise Details
Skullcrusher / Lying Triceps Extension
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Target muscles: Triceps
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Proper form cues:
- Angle the upper arms slightly back (past vertical) rather than pointing straight up at the ceiling
- In the fully extended/contracted position, the forearms should form an angle with the vertical — not be perfectly parallel to gravity
- The bar’s arc should travel back toward the area behind the head, not downward toward the face
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Common mistakes to avoid:
- Setting up with arms perfectly vertical so the forearm is parallel to gravity at full extension — this eliminates tension on the triceps at the top of the rep
- Following the exercise name literally and directing the bar toward the skull or nose, which reinforces the wrong arm angle
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Sets/reps: Not specified in this video