The next time you bonk, blame your brain—not your muscles.

For almost the entire history of the field of exercise science, fatigue was believed to have been caused by “catastrophic” events in the muscles, such as muscle glycogen depletion and lactic acid buildup that caused them to simply stop working right. But a recent trickle-turned-flood of research has demonstrated with increasing certainty that fatigue is in fact caused by a voluntary reduction of brain activation of the muscles. In short, we bonk because we quit.

Proof of this comes from a 2010 study led by Emma Ross, an exercise physiologist at England’s Brighton University who also happens to be a marathoner and Ironman finisher. In this study, eight experienced runners ran a 20K time trial on treadmills. Before and after the time trial, researchers measured the maximal voluntary contraction force of the runners’ quadriceps muscles and the maximal force of the same muscles produced through external electrical stimulation. The runners were also asked to pause at the 5K, 10K and 15K points of the time trial so that the same measurements could be taken.

Maximal voluntary muscle contractions (MVCs) are used to quantify fatigue. The idea is that when a certain muscle such as the quads is fresh and rested, it will produce a certain amount of force when a subject is asked to contract the muscle as forcefully as possible, but after prolonged exercise, the subject will not be able to match that level of force production. The difference between the magnitude of force production before and after exercise serves as a pretty good measurement of fatigue.

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What this measurement does not tell you is the source of the fatigue. The cause of a decrease in MVC could be fatigue within the muscle itself (resulting from something like lactic acid buildup) or it could be caused by the brain’s refusal to contract the muscle as forcefully as it did before exercise. That’s where the other measurement—electrically stimulated maximal muscle contraction force—comes in. The idea here is to bypass the brain by shocking the muscle and seeing how much force it can produce that way. If the MVC decreases after exercise, indicating some kind of fatigue, and the maximal contraction force of the muscle produced through electrical stimulation decrease to a similar degree, then you know that fatigue was caused by a breakdown within the muscles themselves. But if the MVC decreases after exercise and the maximal contraction force of the muscle produced through electrical stimulation decreases only a little or not at all, then you know that fatigue was enforced by the brain.

Guess what happened in this study? MVC held steady until the 15K point of the time trial and then plummeted, indicating that fatigue began at this point (although the subjects held a steady pace throughout the entire 20K). But the maximal electrically stimulated contraction force of the quadriceps muscles held steady throughout the full 20K time trial and beyond. This shows that although the runners rated themselves as exhausted at the end of the time trial, their muscles were perfectly capable of continuing much longer. Their muscles were not fatigued in the least. The reason they could not in fact continue was that their brains had decided enough was enough and reduced their own electrical stimulation of the working muscles.

The brain-centered nature of exercise fatigue is believed to be an evolutionarily developed protection mechanism that prevents us from exercising ourselves to death.

MORE: Exercise & Science in Sports & Exercise


About The Author:

Matt Fitzgerald is the author of numerous books, including Racing Weight: How To Get Lean For Peak Performance (VeloPress, 2012). He is also a Training Intelligence Specialist for PEAR Sports. To learn more about Matt visit