I am 49 years old and have been doing endurance type sports since age 6 and started tracking my HR on the popular devices since 2004, attending every major training coarse I can and reading a few books how to calculate (Max HR - (Resting x 80%) + Resting) = 80% Max HR, etc.
The challenge I have is that my Max HR has never exceeded 155 and the more I train the lower it goes. I recently have to really push extremely hard on a cycle race of 3-4 Hours and if I push excessively high on a 5-7% gradient can get my HR up to 144, but it hurts really bad and at that point I cannot breath and definitely wont not be able to talk.
When doing non-frequent exercises (in my case running or weight training) my Hr go up very quickly, but max out at the same level, when I run out of breath and feels like I am going to die from oxygen starvation.
When doing an ECG (treadmill) the feedback is that my hart is very big as a result of my sporting history and being active since age 6.
I recently spoke to someone during a training ride who mentioned that he had the same problem and through medication had it resolved. This gave me new hope, as I still think it is VO2 related and not the actual Max HR itself.
I used to be an very explosive cycling sprinter in my young days, but the low range between a moderate level ride @ HR 105 to a serious sprint @ HR140 is just not enough to keep up the tempo for the final 2-3 kilometers.