The Frontier X2 measures both, Breathing Rate and Heart Rate. Breathing rate is much more closely correlated with your true internal effort than heart rate, because unlike heart rate, it is not influenced by external factors like caffeine, lack of sleep, temperature, humidity and stress, which are some of the problems with heart rate based training. The Breathing Rate based alerts on the Frontier X2 tell you when to speed up and when to slow down, enabling you to optimize your performance.