I don't think it has anything to do with toughness (although armchair fans love to complain that professional athletes are far less bad-ass or tough than they themselves are). It likely has more to do with physics. Can't remember the equations but essentially muscle strength increases in what is essentially two dimensions (a muscle is as strong as its cross-section). What it has to do (act against) often increases in terms of volume (three-dimensions). (Or with, say, speed, in one dimension) So in certain situations, if you double the 'load' you have to quadruple the size of the muscle. (elephants do not have the same body proportions as do mosquitoes). Probably most noticeable in football, where a small increase in force requires a disproportionate increase in protective equipment. What human bodies evolved to do is deal w/ 'run-of-the-mill' activities. When you get to extremes, the 'ordinary' proportions of humans will no longer be up to the job. And most pro athletes are at those extremes. That's why they get injured more than those of us who sit around drinking beer, as God and Darwin intended.