Start in a push up or plank position. Hands flat on the floor and below your shoulders. Legs extended behind you. Your body should be straight. My model's butt is a smidgen high in the air.
Bend your right leg and bring it toward your chest, then straighten it back to the starting position. Repeat with your left leg for one rep. Alternating legs this way, do as many as you can in 1 minute being sure to keep good form, keeping your hips and butt low.
If you go to the pool, try doing these in the water using the opposite arm of whatever leg you are moving as if you are climbing a mountain face down. I love doing these in the water!