For Knees to Elbows, start as if you were about to do a push up. Toes on the ground, hands directly under your shoulders on the ground, butt level with back. Then move your left knee to your left elbow. You should be able to feel it in your abdominal muscles. Bring it back to plank position, and repeat with the other leg, bringing your right knee to your right elbow.
Allow your body to move back and forth as you’re doing this.
Make this easier by putting your hands on a chair or a box instead of the floor, and just bring your knees up as far as you can.
Make this harder by elevating your feet on a box.