Lie on your back with your arms along your sides and raise both legs off the floor, holding them low and straight. Open your legs slightly apart, then cross one over the other, and keep alternating which leg is on top with every cross. The lower your legs travel, the harder your abs have to work to keep your lower back pressed into the floor - and pressed it must stay, for every single rep.

Works: abs, obliques, hip flexors, inner thighs

Breathing: Steady and rhythmic, in time with the crosses - don't hold your breath as the set drags on.

Common Mistakes: Letting the lower back arch off the floor, bending the knees as fatigue sets in, and holding the legs so high the abs stop doing any real work.

Make Easier: Raise your legs higher toward the ceiling, tuck your hands under your hips for support, or build up with Flutter Kicks first.

Make Harder: Hold your legs lower to the floor and slow every cross right down, or move on to Windshield Wipers for a bigger rotational challenge.