Jackknife

How to do it

1. Lie flat on the back on the floor with the legs straight and the arms extended overhead.
2. Brace the core and simultaneously lift the straight legs and the upper back off the floor.
3. Reach the hands toward the shins at the top of the movement, folding the body into a V shape and working the core through full contraction.
4. Exhale sharply as the body folds up and inhale while lowering back down.
5. Lower the legs and torso together under control until they hover just above the floor without resting.
6. Keep the lower back pressed toward the floor at the start of each rep to avoid arching as the legs descend.

Muscles worked

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