Compatible with iPhone 4S and 5 and Android Bluetooth Smart Ready devices running Android 4.3 including the Samsung Galaxy S4 * Allows heart rate data transmission without additional adapters via Bluetooth Smart technology * Works with top-rated Runtastic Smartphone Apps for complete workout stats including heart rate zones and calories burned * Transmits heart rate to indoor cardio equipment at health clubs and home gyms using 5.3 kHz technology * Very low energy consumption, with a long-lasting battery that is easy to replace, and washable belt