Lie on your back. Rest your hands by your sides, palms facing down, or place your hands underneath your butt for a little leverage.
Extend both legs out so they are a few inches above the ground. Draw your lower abs in, pressing your lower back into the floor.
In one fluid motion, bend the knees into the chest, press the hips up as high as you can, lower your hips to the floor, and extend the legs straight out.
This completes one rep.
Do as many reps as you can in 40 seconds, then take a 20-second rest.