Lie flat on your back with your legs straight and your arms along your sides, palms down, head and shoulders lifted slightly. Keeping your legs straight and together, raise them up to about 45 degrees, then lower them back down until your heels hover just above the floor - and raise them again without touching down. The lower back is the referee: it stays pressed into the floor for the entire rep, and if it starts to arch, your legs have gone lower than you've earned yet.

Works: abs, hip flexors, quads

Breathing: Exhale as you raise the legs, inhale as you lower them.

Common Mistakes: Letting the lower back arch off the floor on the way down, dropping the legs with gravity instead of lowering them, and bending the knees more and more as the set goes on.

Make Easier: Bend your knees slightly, shorten the range, or slide your hands under your hips - or build up with Reverse Crunches and Flutter Kicks.

Make Harder: Slow the lowering to a three-count and keep the hover longer, or move on to Scissors (Legs) or Butt-Ups.