Marcora believes that this limit is probably never truly reached—that fatigue is simply a balance between effort and motivation, and that the decision to stop is a conscious choice rather than a mechanical failure
Perhaps, mechanical failure is also possible. Extremely strenuous exercise can actually cause muscle tissue to break down and enter the bloodstream, resulting in rhabdomyolysis, a potentially lethal kidney condition[1]. This was quite rare, but recently many cases of it have happened to people who do Crossfit.[2]
[1]http://en.wikipedia.org/wiki/Rhabdomyolysis [2]http://www.huffingtonpost.com/eric-robertson/crossfit-rhabdo...