Mind over Muscle or Where is Fatigue?

The old theory, taught to me in high school, is that muscles become fatigued when they run out of fuel/oxygen or they become suffused with lactic acid, an unpleasant byproduct of work. But if this is so, why do athletes almost always manage to go their fastest in the last mile of a race when their muscles should be closest to exhaustion? An article in New Scientist, (“Running on Empty,” by Rick Lovett, 20 March, 04, p.42-45, a copy is here) based on the work of Timothy Noakes and others, raises some more puzzles and offers a new theory.

If fatigue is based in the muscles then without more fuel, oxygen or less lactic acid you should not be able to improve performance. Yet, amphetamines and drugs like Ecstasy do allow athletes and clubbers to work and play harder (sometimes to dangerous effect). Measurements of the input factors also show that (absent unusual factors) fatigued muscles don’t in fact run out of critical factors.

The common sense response to these puzzles is that runners speed up in the last mile because they know it is the last mile and are willing to push themselves to their limits. Similarly, drugs fool the brain into thinking that the muscles are less fatigued than they are. If one thinks seriously about this common sense notion, as has Thomas Noakes, it provides a quite different view of fatigue than the old theory. The brain in this view is a central regulator that monitors the muscles and sends out messages of fatigue, quite possibly long before the muscles are truly spent as a sort of insurance policy. The central regulator theory doesn’t say that fuel and oxygen are unimportant only that the relation between fuel and fatigue is mediated by the brain.

The central regulator may have rational expectations. Experienced runners apparently report that the first mile of a 10k race is easier than the first mile of a 5k race. Makes no sense on the old theory but if you think about the central regulator meting out a fatigue budget in advance then everything becomes clear.

How then to improve performance? Try convincing yourself that you are running a 10k instead of a 5k (hypnosis may work). Also, Noakes suggests interval training, interspersed bouts of high intensity workouts with recovery breaks. The idea here is to the teach the central regulator that going faster won’t do you any harm.