Lie on your back with your arms by your sides and lift your head and shoulders off the floor. Raise both legs, then pedal them in the air: bring one knee in toward your chest as you extend the other leg out straight, keeping it off the floor. Keep switching legs in a smooth, continuous cycle, with your lower back pressed into the floor throughout.

Works: abs, obliques, hip flexors

Breathing: Exhale as you draw each knee in, inhale as you switch - keep the rhythm steady and never hold your breath.

Common Mistakes: Letting the lower back arch off the floor as the leg extends, pedaling so fast that momentum takes over from your abs, and dropping your head and shoulders back down between switches.

Make Easier: Extend your leg higher up rather than out - the closer your straight leg is to vertical, the lighter the load. You can also rest your head down and keep the pedaling motion only.

Make Harder: Extend your leg lower so it hovers just above the floor, slow the cycle down, or move on to Scissors (Legs) or Windshield Wipers for a bigger challenge.