Peter and Lynn,
I think you are both facing what is the heart of Jun's question. If it was really 'safe' to push ourselves arbitrarily close to our real physical limits, the question would be easy. But it's not really safe, and so each of us has psychological limits that we develop (or are born with) to protect ourselves. Sometimes, these psychological limits are over-protective and, from the outside, it is clear that a person could go well past the limits they perceive. This is when the temptation to push a person 'past their limits' comes up for us. Like I said earlier, I believe it is healthier to respect the limits -- psychological or physical -- because it is better to work with what is their than to work with what ought to be there. Still, I'm not stranger to the compelling logic of the other point of view.
In fact, sometimes I wonder whether this acquiescence to working within limits is an over-protective psychological limit that I impose on myself.