SETUP: Lie flat on the ground and grab hold of something very secure behind your head. A neutral-grip works best. Keeping your feet together and a slight bend at the knee.
LIFT: Tighten your abs and breathe out as you raise your legs up off the ground and draw your knees to your chest. Make sure you get your legs beyond 90 degrees, otherwise your hip flexors are working harder than your abs, and that’s not the goal. Draw your knees in toward your chest and squeeze your abs hard. Your butt should be off the ground at the peak of each rep.
TIP: Breathe out as you lift your legs and squeeze, then breathe in, then breathe out as you lower your legs under control, and breathe in before you start the next lift. This will enable you to really feel the target muscles working.