Lie on the floor (or a mat) with your hands under your butt, palms facing up. Engage your core, raise your head off the floor, and keep your legs straight as you lift them both off the ground to 90 degrees so they’re pointing straight up at the ceiling. Lower your legs back down, keeping them straight the whole time. Do not let them touch the floor and repeat.
Make this easier by letting your feet touch the ground and taking a short rest between reps. Or by bending your knees completely and bringing your legs up to your torso, then straightening them out again as one rep.
Make this harder by resting your arms on the floor straight out from your sides at a 90 degree angle from your body, palms facing up.
This exercise is used in our rock climbing training plans, available at trainingbeta.com.