Don't get an ab wheel. Get a Power Wheel. Roll outs aren't a core exercise. While they do work the abs heavily it is a full upper body exercise. The plank is not the fundamental core stability exercise. It's too easy and not unstabilizing. At most it's a transition exercise. Once one can hold it for a minute it's time to move to harder variations. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-7wbYKnLWr4I.htmlsi=n18MInPV5ocjamg_
