Every marathon runner dreads "hitting the wall," or being overwhelmed by fatigue before the finish line is within reach, forcing you to slow way down or even stop. Different things cause this in different runners and most runners assume they know what hitting the wall is.
Muscles run out of glycogen--their favorite fuel--and as a result, there is no longer enough energy available to hold pace. Or perhaps muscles produce too much lactic acid, which builds up to the point where the muscles stop working properly. Or maybe the runner becomes too dehydrated and fatigue results from heat accumulation in the muscles.
Whatever the specific cause, hitting the wall in a marathon--or any other run--is the result of some type of functional breakdown within the muscles that impairs their ability to function at the desired level. Right?
Wrong. Cutting-edge research in exercise physiology demonstrated that fatigue is seldom, if ever, caused exclusively by events that occur within the muscles. Fatigue is actually caused by the brain, which reduces its electrical stimulation of the muscles and produces feelings of discomfort to prevent any real damage from happening to the muscles or other organs.
The main problem with the old muscle-centered model of exercise fatigue is that the various forms of muscle breakdown--that have been proposed as direct causes of fatigue--never actually occur. For example, research has shown that there's always glycogen left over in the working muscles of "exhausted" athletes. If total glycogen depletion ever did occur, the result would be muscle rigor--the working muscles would become locked in a state of full contraction and the athlete would be essentially paralyzed.
Another proposed cause of catastrophic fatigue is muscular acidosis. The muscles produce lactic acid faster than they can metabolize it and as a result the muscles become too acidic to function properly. If muscular acidosis were a direct cause of fatigue, then fatigue in any given runner would always occur at the same level of lactic acid concentration in the blood. But it does not. For example, studies have shown that with increasing altitude fatigue occurs at lower and lower blood lactate concentrations.
Also, if exercise fatigue were caused within the muscles then fatigue would occur despite a constant or perhaps even increasing level of stimulation from the brain. The brain would continue to shout "Move!" at the muscles, but the muscles would not move because they couldn't. But this is not what happens.
A number of recent studies have clearly demonstrated that fatigue-related declines in exercise performance are almost always associated with corresponding declines in motor output from the brain. Other studies have shown that muscles are able to continue working beyond normal endurance limits--without suffering any harm--when the motor centers of the brain are artificially stimulated.
This is not to suggest that the muscles have nothing to do with fatigue. The brain's decision to reduce muscle activation and prevent damage is based on a constant stream of feedback from the muscles and other parts of the body.
There are mechanisms that enable the brain to continuously monitor muscle glycogen levels, muscle pH balance, core body temperature, dehydration, muscle damage and other important factors that could lead to irreparable harm if exercise continued too long. When one or more of these signals informs the brain that a problem is imminent, the brain responds by reducing muscle activation and producing feelings of fatigue.
Training improves your fatigue resistance largely by raising the threshold for these warning signals. For example, training increases the running pace you can sustain without losing pH balance in your muscles. As a result you may improve your 5K race time--not because you are able to run faster before your muscles become too acidic to function, but because you can run faster before your brain senses an unacceptable increase in muscle acidity.
All of this is very interesting. But does it have any practical implications for your training? Is it possible to use the new, brain-centered model of exercise fatigue to train better? I believe so. In fact, I have written an entire book about the practical implications of this new science, called Brain Training for Runners.