The key point in reading this article is to understand how hard it is get to your max heart rate. I am an experienced runner (sub 2.25 marathon in my youth). Currently 55 years old with a theoretical max heart rate of 220-55=165. In training I sometimes get my heart rate to 165 for brief periods (30-60 secs) BUT in races I can keep it at 165 for 20 mins or so, meaning that 165 is NOT my max heart rate.
So...many of you are probably underestimating your max heart rate and consequently your 70% heartrate.
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Philip Huxley
