Jhbate/add activity

a starting point
Better than METs is probably the [borg scale](http://en.wikipedia.org/wiki/Borg_scale), also self-reported. This advice brought to you by my dad, whose research for many years was in exercise physiology :)

Note the scale: "The seemingly odd range of 6-20 is to follow the general heart rate of a healthy adult by multiplying by 10. For instance, a perceived exertion of 12 would be expected to coincide with a heart rate of roughly 120 beats per minute."

According to my dad, the correlation between self-reported borg and HR has been proven to be so good in research that's its pretty pointless to track your HR during exercise unless you want a fine-grained view of it.