Circuit 2, Exercise 1: 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 your slowly peel your back down, vertebra by vertebra, until your shoulders touch the mat last. This counts as one rep. Complete 10 reps. Image Source: POPSUGAR Photography