Runner's Crunch Start on your back with your elbows resting on the floor at a 90-degree angle. With your core engaged, roll up to almost sitting while bringing your right knee up to meet your left elbow. It should feel a little like running. With control, straighten your leg as you slowly lean your back down, vertebra by vertebra, until your shoulders touch the mat last. Alternate with your other leg to complete one rep. Image Source: POPSUGAR Photography / Kathryna Hancock