Pushing beyond the pain

It is one of the secrets of elite athletes: to keep going at a level of effort that seems impossible to maintain.

The question is, how do they do it? Can you train yourself to run, cycle, swim or do another sport at the edge of your body's limits, or is that something that a few are born with?

Sports doctors say that, at the very least, most people could do a lot better if they knew what it took.