> At this age (well, at every age) it is imperative to get enough rest between exercise days. I've heard it said that there is no such thing as over-training o

> At this age (well, at every age) it is imperative to get enough rest between exercise days. I've heard it said that there is no such thing as over-training only "under resting". > The heart rate variability (HRV, there are apps for that) is a good indicator. Can you give more details on this? How does it work, what number(s) to look at, etc.