Lie on your back and lift your head and shoulders off the floor, arms extended straight along your sides, hovering above the floor. Raise your legs with your knees bent at 90 degrees, shins parallel to the floor. Hold this position and pump your straight arms up and down in small, brisk beats - as though patting the air - while everything else stays perfectly still. The classic set is a hundred beats, which is where the name comes from.

Works: abs, hip flexors

Breathing: Breathe in for five arm beats, out for five - the rhythmic breathing is half the exercise.

Common Mistakes: Letting the lower back arch off the floor, pumping from the shoulders so the whole torso rocks, and holding the breath instead of pacing it with the beats.

Make Easier: Rest your head down, or keep your feet on the floor with knees bent - the arm beats and braced abs still do their job.

Make Harder: Straighten your legs and lower them toward 45 degrees while keeping your back pinned down, or move on to Scissors (Legs) or Crunch Kicks.