Start in a push-up position with your hands under your shoulders and your body in a straight line. Draw one knee slowly in toward your chest, keeping your hips level and your shoulders stacked over your hands, then return the foot to the start and bring in the other knee. This is the climber with all its hiding places removed - no bounce, no rush, just your core holding a plank while one leg at a time travels through it.

Works: abs, hip flexors, shoulders, quads

Breathing: Exhale as you draw the knee in, inhale as the foot returns.

Common Mistakes: Piking the hips up as the knee comes in, shifting the shoulders back behind the hands, and letting the moving foot drag or tap down mid-rep.

Make Easier: Bring the knee only halfway in, take a breath in plank between reps, or drop to your knees between sets to shake out.

Make Harder: Hold each knee at your chest for a 2-count, or pick up the pace and move on to full-speed Climbers.