Each arm warmer is powered by an 8.000 mah power bank and designed to go anywhere to keep a pitchers arm warm and loose before a game, between innings, and for days after you pitch. The deep penetrating therapeutic heat along with a proper routine will increase performance, and can reduce chances of Injuries. Choose the one for you or get both. BEST OF ALL (Arm warmers come with a free 2 year heater replacement warranty). It will last for years! Shipping is free!!! See our Blog page for more Information.