You've never trained this way. Nobody has. The Under Armour Armour39™ is the first-of-its-kind performance monitoring system. It literally tells you how hard you worked. Simply plug the Armour39™ module into the chest strap and workout like you normally do. The Armour39™ module stores your biometric info, including heart rate, calories burned, real-time intensity, and WILLpower™, then transmits it automatically in real time to the Armour39™ mobile app or watch. Compatible only with iPhone® 4S, iPhone® 5, iPod Touch® (5th generation), iPad® 4, and iPad® Mini. Highly responsive and designed to keep contact with the body so you get precise measurements no matter what you wear it to do. Stores up to 16 hours of workout information on the module and transmits within 15 feet of your display device. Module is sweat resistant (cannot be submerged in water). Armour39™ chest strap wicks sweat and stays put. Sensing pads detect the body’s biometric info and deliver it to the module. Made of 76% nylon/24% spandex and adjusts to fit a ribcage measuring 27”-52” around.