Lie on your back with your hands lightly behind your head and your legs raised off the floor. Crunch up and rotate, bringing one elbow across toward the opposite knee as that knee draws in - while the other leg extends out straight, hovering. Switch sides in a smooth pedaling rhythm: elbow to knee, extend, elbow to knee. The rotation comes from your torso, and your lower back stays pressed into the floor throughout.

Works: abs, obliques, hip flexors

Breathing: Exhale as elbow meets knee, inhale through the switch.

Common Mistakes: Yanking on the head to close the distance, flapping the elbows in instead of rotating the torso, and letting the lower back arch as the leg extends.

Make Easier: Slow the rhythm down and keep the extended leg higher, or build up with Crunches and Air Bike Crunches.

Make Harder: Pause for a beat at every elbow-knee touch and extend the free leg lower, or move on to Butterfly Sit-Ups or Sitting Twists.