"How far and how fast can humans go? What defines a person's limits?"
These are some of the questions athlete and sports journalist Alex Hutchinson tries to answer in his book Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance
Is it all down to the fuel you consume before and during exercise, or is it the oxygen levels in your blood, or the size of your muscles or maybe it's the way you train? Or is it all down to the way you think?
There's a fascinating distinction that researchers are making effort and pain, Hutchinson says.
For example, when you're running, you might think you're trying your best but you're slowing down because it hurts too much - but that's not the case.
"If I reach the point where I can't keep pedalling or I can't keep running, it's not because my muscles can't go any more.
"If you were to sneak up behind me and fire a gun in my ear I would be able to sprint away for a bit more.
"I've just reached the point where the effort is too great relative to my motivation."
And because its the brain that's assessing that level of effort, it means that you can get it wrong or you can manipulate it, he says.