Most serious athletes already know about this (in fact, on that same site: http://quantifiedself.com/2011/03/personal-data-visualizatio...). Keeping a record of your resting heart rate (plus blood pressure, temperature, etc) every morning will tell you many things. For example, I managed to get my resting heart rate down to mid 40s when I was training regularly (and heavily). Now that I'm more sedentary, my resting HR is in the mid 50s to low 60s, and I definitely don't feel as good or make as good times. Spikes while in training were also usually indicative of overtraining. It's also mentioned in "The Hacker's Diet" that if you weigh every day, but pay attention to the moving average it can be much more encouraging.