Start in plank with your shoulders over your hands. You can set the dumbbells to the side or place them under each shoulder and grip them with your hands.
With your core pulled in toward your spine, bring your left knee forward toward your chest. Quickly switch legs, bringing the right knee forward while moving the left leg back. This completes one rep.
Keep switching legs and begin to pick up the pace until it feels a little like running in place in a plank position.
Do as many reps as you can in 45 seconds while maintaining proper form. You can modify the movement by slowing down the pace.