The above post is very helpful for me. I've only recently begun to monitor my HR and am trying to make sense of my current data. As a 39 year old using a different formula than the old one mentioned above, I arrive at a MHR of 186. With my zone set at 75%-85%, I cant seem to stay in that range. I've been riding on a trainer and before I purchased the monitor, I was riding 30-40 minutes at about 20mph. Now if I try to stay even close to 85%, I have to stay close to 17mph or my HR goes "too high". Was I previously riding at 90% of HR or, as suggested above, is my MHR simply higher than the formula indicates. Any ideas or suggestions would be appreciated.