Start in a forearm plank with your elbows under your shoulders and your body low. Sweep one knee out and up along your side, drawing it toward your elbow - your hip rolling open as the knee travels - then extend the leg back and sweep with the other knee. Stay low to the floor throughout, like a crawl performed on the spot: the knee hugs the ground on its way up, and your shoulders stay steady over your elbows.

Works: obliques, abs, hip flexors, shoulders

Breathing: Exhale as the knee sweeps up toward the elbow, inhale as the leg extends back.

Common Mistakes: Piking the hips up to make room for the knee, dragging the foot lazily instead of driving the knee toward the elbow, and letting the whole torso sway side to side with each sweep.

Make Easier: Shorten the sweep - knee toward the ribs is enough - and slow it right down, or build the pattern with Slow Climbers.

Make Harder: Pick up the pace with the hips pinned low, or take it traveling with Army Crawl or add the reach with Climber Taps.