Lie back with your head and shoulders lifted off the floor and your arms extended along your sides, palms hovering or lightly touching the floor. Tuck your knees in toward your chest, then kick both legs out straight so they hover above the floor - and pull them right back in. Your upper body holds its lifted crunch position the whole time while your legs fold and extend, and your lower back stays pressed into the floor.

Works: abs, hip flexors, quads

Breathing: Exhale as you kick the legs out, inhale as you tuck them back in.

Common Mistakes: Letting the lower back arch off the floor as the legs extend, dropping the head and shoulders down between reps, and kicking so low the legs touch down.

Make Easier: Kick your legs out higher rather than low, or let your heels rest on the floor briefly between reps - or build up with Crunches and Flutter Kicks first.

Make Harder: Extend the legs lower and hold the kicked-out position for a beat, or move on to Air Bike Crunches or Hundreds.