I recently got a Garmin Forerunner 935 (advised by a good friend of mine). After using it for two months, I can say that I’m happy with it so far. It has lots of functionalities and analytics that I’m still exploring. A lot of them are actually developed by Firstbeat. Firstbeat is a company that develops analytics to understand and monitor stress, exercise and sleep based on heartbeat data. For example, we can find on the Garmin watch estimates of VO2max, aerobic and anaerobic training effects, training load and status and advice on the recovery time.

I was wondering how they can derive all these metrics based on heart data only knowing that, for example, VO2max is obtained using costly metabolic cart measuring for the ventilation and the concentration in oxygen and carbon dioxide of the inhaled and exhaled air. Firstbeat has actually patented an estimation method which is based on anthropometric data (age, gender, height, weight, etc.), heartbeat data (internal workload), and a measure of external workload such as the running speed obtained from the watch GPS.

If one wants to dig deeper, Firstbeat has published a couple of whitepapers to explain the construction of their metrics, and links to scientific papers related to the physiological grounding of their methods. It can be found there. They are particularly useful to understand the somewhat arbitrary scale of their training effect indicators (in [0,5]), what is the EPOC (Excess Post-Exercise Oxygen Consumption) and how it relates to the recovery period.

One should definitively have a thorough look at them to make the most of the watch. Let’s see if I can improve significantly my trail running skills thanks to the Garmin watch and its analytics!