How to do it: Sit with your feet lifted off the floor, knees bent into your chest, hands clasped behind your head. Lean back and extend both legs straight out, touching your back to the floor but keeping your head and shoulders off the ground. Sit back up and return to your start position. That's one rep. Repeat as many times as you can in a row for one minute.
Tip: Be sure to brace your abs during this entire move and try not to use momentum to come up and down. If it’s too tough to do with the legs extended, keep your knees bent as you lower and sit up.