Well I just got back from Northern Arizona and Southern Utah.  I spent 10 days there on vacation, but all my vacations have some exercise in them.  I did quite a bit of running, off road motorcycle and mountain biking.  All was done between the altitudes of 5000-8000 feet. 

The best day was a ride of 31 miles singletrack.  The trails were called Hurricane Rim Trail, Gould's Rim trail, and the J.E.M. Trail.  I also had to ride to the starts.  The actual trails are said to be 21.5 miles.  There was about 2500 ft of elevation gain.  I can honestly say this is one of the best rides I have ever been on.  But I should have brought more fluids.

Anyway after training that long at that altitude, my heart rate has been low versus my perceived effort this week.  For example when running my heart rate is normally in zone 4, about 148 to 155.  This week for the same effort it has been around 138.  Same with cycling.

It will be interesting to see how long it lasts.