This log is used to track my fitness as measured by a Polar heart rate monitor's OwnIndex value. This is theoretically a measure of overall fitness. I'll track it from time to time and see what happens. I don't care much what the number itself is... it's not nearly as accurate as a full VO2Max test. But it is a cheap and easy benchmark to get some sense of how my overall fitness is progressing. The main thing to keep in mind is that the only data the watch can collect is heart rate.