The Central Governor Theory and Racehorses
Fatigue is an interesting concept, especially as it relates to racehorses. Over on Moxy Monitor they have outlined the two standing theories on fatigue, the traditional Cardiovascular/Anaerobic model proposed by A. V. Hill and associates where a lack of oxygen to the working muscles limits exercise performance, and the more recent Hill/Noakes Central Governor model, developed by exercise physiologist Dr. Tim Noakes, where fatigue is generated by the brain as a protective metabolism, i.e your brain regulates your bodies effort to make sure that you only push it so far out of homeostasis.
Fatigue of course is not an absolute event in that we continue moving, albeit at a slower pace. Noakes' Central Governor model works on the theory that the brain monitors the conditions of all the systems in the body and calculates the metabolic costs of continuing at current pace. If the brain thinks that it is about to "redline", that is, reach an absolute limit where it is in danger of a catastrophic event, it tells the body to slow down to a point where it can, not so much recover, but at least deal with what is being thrown at it.
So how does the Central Governor Theory relate to horses when they are asked for a full out effort. Horse trainers, and at some level handicappers, seem to have a decent answer there, especially when it comes to elite runners. Racehorses, it seems, do know their physiological limitations when they run, however in many cases the truly elite horse will, under certain circumstances, competitively push themselves to beyond what they would normally do, effectively 'redlining' themselves. A good example of this was Rachel Alexandra in the Woodward Stakes G1 against older horses. After being pressured both sides and setting a furious pace Rachel Alexandra was able to push off those two foes only to be met by Bullsbay who had made little effort in the early stages of the race and was fresh on the scene. Rachel Alexandra pushed off Bullsbay at the top of the straight, was now asked for an all out to lift again and win the race when Macho Again, who had also done no work up to that point, came with his run. It was a truly all out effort for the entire race. Sensibly, recognizing it was a supreme effort, her trainer put to the side for the year but while she won a couple of races following that event, she was never the same horse again. Holding to the Central Governor Theory, her brain recognized her physiological limit and never allowed her to get near that point again.
In quite a few interviews leading Irish trainer Aiden O'Brien references never wanting to find "the bottom" of a horse in training or a race, talking about the same concept we have illustrated above. Keeping a horse competitive, but not having it truly reach its physiological limit, is what trainers do every day, but the truly elite horses seem to have the ability to push that limit a little further than most.