Start in a high-plank position with your shoulders over your hands and weight in your toes.
With your core engaged, bring your right knee forward under your chest, with your toes just off the ground. Quickly switch legs, bringing your left knee forward, keeping your shoulders over your wrists and your back flat.
Keep switching legs and begin to pick up the pace until it feels a little like running in place in a plank position.
Continue for 30 seconds.
Rest for one minute. Complete circuit two one more time, then rest for two minutes before starting circuit three.