Start lying on your back, core pulled in and lower back flat against the ground. Reach your arms straight to your sides and lift them off the floor. Lift your legs off the floor at about a 45-degree angle, keeping them straight. Lift your head so your shoulders are off the floor as well, focusing the movement in your core so you don't strain your neck.
Use your core to lift your upper torso off the floor as you simultaneously bend your knees. You can lean back to make this move harder or come up more to make it easier.
Slowly lower your upper body back down to the floor as you straighten out your legs. Return to the starting position, with your back on the floor and your head, shoulders, and legs lifted.
This completes one rep. Complete as many reps as you can with proper form in 20 seconds.