SG, the heart's a strange organ being that all it is, is a pump. The whole machine can be fine but for one worn bearing.
The one thing I know about hearts and running is that at maximum rates the heart pumps less blood volume than at slightly less. This is because at maximum hr's, the blood flow become turbulent and inefficient, which subsequently results in the blood lactate being cleared less efficiently.
From there we quickly achieve knackerdom.
Good news for those of us who don't really feel like giving 100%.
I knew a guy who looked like Clark Kent; or was it Harry Potter? who only played football so he could kick people. The idea of someone thinking they could make him yield ground because he was slower than them doesn't bear thinking about.
When he used a cash point, he had this 'come on, just try mugging me' attitude to anyone nearby. He was a full blown psychopath,