I was thinking about this the other day, and was curious to know when everyone (who trains with an HRM) recalculates their heart rate training zones. Is it a monthly thing, yearly thing etc... I know it would depend on the method you used to calculate these zones in the first place, but obviously as you get older or fitter , your RHR and MHR would change, I assume. Any takers? I did search for this before posting, but couldn't find much.