Start in a push-up position, hands under your shoulders. Jump both feet forward together, landing in a crouch with your knees tucked up toward your chest and your hands still planted - then jump both feet straight back out to the plank. In, out, in, out: your abs pull the knees in, your hands never move, and your hips stay low on the way back so you land in a plank, not a pike.

Works: abs, hip flexors, shoulders, quads

Breathing: Exhale as you jump the feet in, inhale as you kick back out.

Common Mistakes: Landing the jump-in flat-footed and far from the hands, piking the hips high on the way back out, and letting the shoulders drift behind the hands as fatigue builds.

Make Easier: Step one foot in at a time instead of jumping, or shrink the jump so the knees come in halfway - or build up with Climbers and Plank Jacks.

Make Harder: Speed up the in-out cycle without letting the hips rise, or move on to Burpees or Ape Hops.