The Science of Sport blog has taken on the topic of "fatigue". And not just taken it on, but challenged it to a month-long grudge-match whereby only the last man will be standing. They've already written three great posts setting up the debate, which will center around a biochemical explanation for fatigue (i.e. muscles get tired/leaky/lactic/etc, causing people to slow down) versus what they call "anticipatory regulation" (i.e. the brain forces the individual to pace themselves to handle the task at hand -- also known as the Central Governor model of exercise, though the authors choose not to use that terminology due to its implication of a "black box" part of the brain that does all of the governing).
They've gone through a couple examples of pacing and why it should be strange that an individual is able to run faster at the end of a race. They've also discussed leaky calcium channels, performing in the heat and a few other scenarios. Now's a great time to get yourself caught up on the series because they're about to get into the nitty-gritty.
And in case you haven't read their blog, Ross and Jonathan are recent PhD grads who studied exercise physiology and this is basically their dissertation. So the content promises to be thorough and the views passionate. I'll be sure to direct you to anything particularly interesting from an Optimal Training standpoint, but by all means you should check it out as there is quite a lot being covered.
(Graphs from part 1A)