Place both hands on the ground, shoulder-width apart. Fully extend your legs so that your body is in one straight line from head to toe. You should be resting on the balls of your feet. Pull your belly button into your spine to engage your core. This plank is your starting position.
Keeping your left foot on the floor, bend your right knee and bring it in toward your chest. Then extend your right leg to return to the starting position.
Keeping your right foot on the floor, bend your left knee and bring it in toward your chest. Then extend your left leg to return to the starting position.
This is one rep; continue alternating.
As you become more familiar with the movement, begin to pick up the pace. It should feel like you're running in place. Don't forget to drive your knees to your elbows, always picking your feet up and off the mat.