Start in a low lunge position with left knee bent, right leg straight behind you, and arms extended overhead. Hinge forward slightly at hips, so chest is over left thigh.
Use your core to lift right knee toward chest and allow arms to come in at sides. Replace leg on floor and, as quickly as possible, drive forward again. Do 20 reps, then switch sides.
Make it harder: For more of a challenge, add a twist, pulling right knee in to touch left elbow each time.