Yeah, what's the deal? You aren't actually a "training machine" You just can't go all out and get to MHR?

The "heartrate" statistic of a given person is part of a "cardio "vascular" "" feedback loop.

Remember the "vascular" part? That's the part of the "loop" that carries oxygen, energy and the "feedback" signals to the heart to work harder and faster.

Depending on the status of your muscles and energy supplies in you blood, you may not have enough "feedback" for the heart to work any harder.

If you want to see how hard you can make your heart work, you have to rest between efforts.....