Start in a traditional plank with your shoulders directly over your hands and wrists. Be sure to keep your back flat and your butt down, maintaining a neutral spine.
Engage your core (think about pulling your belly button toward your spine) and lift up your right knee, bringing it toward your elbow. It's OK if you can't bring the knee all the way to your elbow. Return the right knee back to the starting position as you simultaneously drive your left knee up toward your left elbow. Return to the starting position.
Repeat with your left leg. Continue switching legs for 30 seconds.