This simple-yet-challenging move targets your core and upper body.
To do it, come into a plank position on the balls of your feet, with your palms flat on the floor and your wrists below your shoulders. Keep your head in line with your spine (don't let it drop) and contract your abs.
Hold the plank while bringing your right knee under your torso. Return your right foot to starting position and switch legs, lifting your left knee under your torso. Keep switching legs for 20-30 seconds. Rest, and repeat. As you improve, work up to doing the move for 1 minute. Rest, and repeat up to two more times.
The faster your pace, the more of a cardio exercise (and a challenge!) this move will be.