Runner's Crunch Lying flat on your back, rest your elbows at a 90-degree angle on the floor. Keep your legs extended out in front of you. With your core engaged, roll up by bringing your shoulders off the ground, followed by each vertebra until your lower back is off the floor. At the top of your roll-up, bring your right knee up to meet your left elbow. With control, as you straighten your leg, slowly peel your back down, vertebra by vertebra, until your shoulders are the last to touch the floor. Alternate with your other leg to complete one rep. Image Source: POPSUGAR Photography