Stand tall and raise one knee up until your thigh is at or above parallel with the floor, fists up in front of you in a guard position. Hold it there. Your standing leg keeps you steady, your abs keep you upright, and your raised leg works to keep that knee from drifting down - which it will try to do almost immediately. Keep your torso tall and still, complete the hold, then switch legs.

Works: abs and obliques, quads, glutes, calves

Breathing: Breathe slowly and evenly - a steady rhythm helps you stay balanced.

Common Mistakes: Letting the knee drop below parallel as the hold goes on, leaning back to keep the knee up, and wobbling through the hips instead of keeping them level.

Make Easier: Hold the knee lower, or rest a hand on a wall for balance.

Make Harder: Raise the knee higher above parallel, extend the hold time, or close your eyes and let your balance work overtime.