I wasn't a great believer in fitness trackers for athletes, but this little device has changed my mind. For $18 (and a bit of waiting for it to make it from China) you get something that's as good as any of the basic fitness trackers, and even has heart rate monitoring built in.
In this post I will look at some of the common 'thresholds' and empirical evidence for their existence. I'll be using evidence from the world of running - as a sport it is perfect for the purpose; we are well-adapted to it as a species over a wide range of distances (read 'Born to Run'), and we have kept records of the limits of our achievements. Whilst they were not all run by one person, they form a lovely natural experiment into the limits of human physiology.