Physical vs Mental
There is a fine line between our physical and mental abilities.
Our bodies are controlled by our brains, it will carry on if our brain tells it to do so, sometimes beyond where we mentally think we are capable.
When I was training really hard and losing weight, my brain was often the blocker when it came to exercise, I would start to struggle and my brain would tell me that I couldn't do it and want to stop. It takes a lot of brain work to push past that and carry on.
Of course if you are injured it is a different thing and you shouldn't tell your body to push past it's limits if you are in pain. I suppose what I am trying to convey is that we are capable of so much more than we think, if only we would believe in ourselves!