Fatigue is a primary limiter standing between you and better performance. If you could delay or resist the sensations of fatigue you would go faster and last longer at a given effort level–the ultimate purpose of training. Yet we never rid ourselves of fatigue, which is actually a good thing because this prevents us from damaging our bodies or perhaps needlessly expending physiological resources. But understanding what brings on fatigue during a race or workout may point to strategies that could raise your fatigue threshold, allowing you to go faster or farther.
Fatigue seems to vary according to the duration and intensity of exercise. An 800-meter runner and a marathon runner may both fatigue greatly during their races, slow down, and struggle to the finish lines, but their specific reasons for fatigue aren’t the same. Or are they? What caused their fatigue? Currently there are three ways of explaining fatigue.
Catastrophe theory. This is the oldest model having been around since the 1920s. It’s the one which is accepted by most exercise physiologists. This model proposes that exercise stops when something catastrophic occurs in the body, especially in the working muscles [1].
Other than overheating and severe dehydration, which can obviously limit performance, the catastrophe model proposes that there are at least two common physiological reasons for fatigue during endurance events: the accumulation of metabolic by-products such as hydrogen ions, especially from lactic acid release (the 800-meter runner); and the depletion of energy stores such as glycogen and glucose (the marathoner). The catastrophe model proposes that when either of these situations occurs the body is forced to slow down. It’s much like a car running out of gas or the fuel lines becoming clogged. A catastrophe has just happened and the body stops functioning normally.
Central Governor theory. The second way of explaining fatigue originated in the physiology lab at the University of Cape Town in South Africa in the 1990s. Here noted-exercise physiologist Tim Noakes, PhD proposed that fatigue occurs in the brain, not in the muscles [2,3,4].
In this model the body is constantly sending signals to the subconscious brain regarding the current status of the working muscles. For example, fuel levels and metabolic by-product build-up are being monitored by the brain. This is a bit like the operation of the thermostat in your home which gauges the temperature and turns the heating or air conditioning system on or off as needed. At some point the brain may make a decision, again subconsciously and the result of perceived exertion, to slow down due to the current status of the body. It’s proposed that this central governor for fatigue evolved to protect the body from damage caused by excessively hard work.
Psychobiological theory. This theory is a bit like the central governor model, but with a twist. Samuele Marcora, PhD at the University of Wisconsin proposed recently that it is, indeed, perceived exertion, a subconscious calculation made by the brain during exercise, that limits performance [5,6]. He proposed that exercise stops well before fuel levels and metabolic by-product accumulation suggests it is absolutely necessary.
In a part of the forebrain known as the anterior cingulate cortex (ACC) subconscious decisions are made regarding conflict resolution and response inhibition. Essentially, this means that during exercise the ACC is weighing the cost of continuing at a given intensity versus the reward for doing so. Dr. Marcora has shown that “fatigued” athletes are able to overcome the sensation at what appears to be the end of exercise to failure and produce a greater output if the reward is big enough.
You have probably experienced this at the end of a race. You may have been slowing down but when the finish line is seen you have the capacity to some how speed up or even sprint. You’re willing to overcome the suffering because the reward, an awe-inspiring finish or perhaps a slightly faster time or higher finishing place, was great enough to overcome the suffering you were feeling. He further suggests that this system evolved to keep us from needlessly wasting energy in the pursuit of food when the prospect of success in finding it was low. But should food appear (perhaps a deer on the horizon) increasing the likelihood of getting it, then the suffering becomes tolerable.
References
1. Davis, J. M., and Bailey, S. P. "Possible mechanisms of central nervous system fatigue during exercise." Medicine and Science in Sports and Exercise, 1997; 29(1):45–57.
2. Noakes, T. D., St. Clair, A., and Lambert, E. V. “From catastrophe to complexity: A novel model of integrative central neural regulation of effort and fatigue during exercise in humans.” British Journal of Sports Medicine, 2004; 38(4):511-14.
3. Noakes, T. D. and St. Clair, A. “Logical limitations to the ‘catastrophe’ models of fatigue during exercise in humans.” British Journal of Sports Medicine, 2004; 38(5):648-49.
4. Noakes, T. D. “Time to move beyond a brainless exercise physiology: The evidence for complex regulation of human exercise performance.” Applied Physiology, Nutrition and Metabolism, 2011; 36(1):23-25.
5. Marcora, S. M., Staiano, W., and Manning, V. “Mental fatigue impairs physical performance in humans.” Journal of Applied Physiology, 2009; 106(3):857-64.
6. Marcora, S. M. and Staiano, W. “The limit to exercise tolerance in humans: Mind over muscle?” European Journal of Applied Physiology, 2010; 109(4):763-70.
