A race should be run at a pace that is sustainable for a long duration. But pacing can be tricky. On a cool, dry day after a good night’s sleep, you could go much faster than your usual, but if you’re recovering from an illness or the day is unusually hot, for example, this will feel much harder.
This is because your brain perceives the effort your body is exerting and not your actual pace. Numerous studies (1, 2) have shown that breathing rate (BR) is linearly related to the rate of perceived effort (RPE) and is a great way to objectively and accurately measure internal effort. This is because both BR and RPE are determined by the same central command signals sent out by the brain to the muscles when you engage in hard exercise, unlike heart rate, which is influenced by various external factors.
By looking at the average breathing rate during your workout session, you can tell how intense the session was. You can even look at what parts of the session you spent in different breathing zones; this is particularly useful if your session had multiple pace changes, especially in interval training.
The Frontier X2 can be configured to alert you when you cross certain breathing rate thresholds, so you can be alerted with a buzz when you are crossing into a higher effort zone than what you had planned for the workout. This can be used to optimize your training in different effort zones.