I keep good records of my rides with average and max HR attained for each ride and I'm having a difficult time keeping my average up. For most of the year I would average 149-152 for a 20 mile ride with a peak of around 168, but lately it's becoming difficult to keep my average above the 139-141 area. I can still manage the max range no problem, and my average speed is actually going up to almost 16 mph (albeit only .5 mph or so) but I can't get the average HR up...am I just in better condition? I just bought Friel's Cycling Past 50 book and I'm about 2 chapters in so I hope to find more info in there. Any thoughts?