How strong is your WILLPower™? The Armour39™ app monitors your workout by tracking your heart rate, calorie burn, real-time intensity, and WILLpower™, Under Armour®'s unique measure that reflects overall workout difficulty in a single score. The Armour39™ app is compatible with any Bluetooth Smart Heart Rate monitor, although optimized for use with Under Armour®'s Armour39™ Strap and Module. This app is compatible with iPhones 4S and later and optimized for 7.0. Continued use of GPS running in the background can dramatically decrease battery life.