Elite athletes can push their bodies to the very limit, but even they can't surpass the boundaries of human nature, according to new research. Scientists have found yet more evidence that, regardless of a person's fitness or training, the human body is limited and can't burn calories at more than 2.5 times its resting metabolic rate for long periods of time. Exceeding this rate when training for 30-plus weeks is not impossible, but it's rare. Previous studies have also found a similar '