Why I do it: Mountain climbers may be basic but when I don't feel like jumping for burpees, mountain climbers are just as effective for getting my heart rate up.
Start in a traditional plank — shoulders over hands and weight in your toes.
With your core engaged, bring your right knee forward under your chest, with the toes just off the ground. Return to your basic plank. Switch legs, bringing the left knee forward.
Keep switching legs and begin to pick up the pace until it feels a little like running in place in a plank position.