Place a Swiss ball in front of you on the floor. Place forearms and fists on the top of it and keep your body in a straight line from your ankles to head. Keep core engaged, elbows bent at 90 degrees, and naturally arch lower back as you roll the ball forward. Make sure your body doesn't collapse as you perform this movement. Pause here, then using your abs, pull the ball back toward knees to starting position. https://i.ytimg.com/vi/WJ6HyT4rCbs/hqdefault.jpg?sqp
[Guideline] Alberti KG, Eckel RH, Grundy SM, et al. Harmonizing the metabolic syndrome: a joint interim statement of the International Diabetes Federation Task Force on Epidemiology and Prevention; National Heart, Lung, and Blood Institute; American Heart Association; World Heart Federation; International Atherosclerosis Society; and International Association for the Study of Obesity. Circulation. 2009 Oct 20. 120(16):1640-5. [Medline].
^ Expert Panel on Integrated Guidelines for Cardiovascular Health and Risk Reduction in Children and Adolescents, National Heart, Lung, and Blood Institute (Dec 2011). "Expert panel on integrated guidelines for cardiovascular health and risk reduction in children and adolescents: summary report". Pediatrics. 128 Suppl 5: S213–56. doi:10.1542/peds.2009-2107C. PMC 4536582. PMID 22084329.