Start in a plank with your hands stacked under your shoulders and your body in a straight line from head to heels. Drive one knee toward your chest, then snap it back as the other knee drives forward, like you're running against the floor. Keep your hips low and level throughout — if they pop up into the air, you're resting instead of working. Move at a quick, steady pace and keep your core braced so your lower back doesn't sag between reps. Your hands stay planted and still; all the movement happens from the hips down.