A skull crusher performed with a low pulley behind your head, so the cable keeps the triceps under tension even at the top, where a free-weight version unloads.
Primary muscles
This information is general fitness guidance, not medical advice. Stop if you feel pain and consult a qualified professional.
Main muscles working (thicker lines)
Lie on a flat bench with your head toward a low pulley, holding a straight bar or rope with the upper arms vertical and the elbows bent so the forearms point back toward the pulley.
Keep the upper arms pointing at the ceiling.
Extend the elbows until the arms are straight over the shoulders.
Hold the squeeze for a second.
Bend the elbows to lower the handle back toward the forehead.
Breathing: Exhale as you extend, inhale as you lower.
Elbows in
Upper arms still
Constant tension
Letting the upper arms drift back
Flaring the elbows
Letting the stack drop between reps
Have someone hand you the bar or pull it into position from a seated start before lying back.