A Matter of Survival

Training to combat fatigue

December 3, 2007
One of my first encounters with fatigue occurred when I had mononucleosis my senior year in high school (that's what I get for kissing that girl). I was so fatigued that I had to sit in the back of the classroom so I could put my head on my desk and go to sleep–forget about running during track practice. Running, even when healthy, however, represents a different kind of fatiguel: an acute, heavy, dead-legged feeling that makes you want to slow down, stop, and sometimes–usually around mile 22 of the marathon–curl up in a ditch and take a nap. Runners are as intimate with fatigue as football players are with bruises or ballet dancers with blistered toes: Fatigue is an entrance fee to success in our sport.

It's also a hot topic of scientific research. From a physiologist's perspective, fatigue is the inability to maintain or repeat a given level of muscle force production, resulting in an acute impairment of performance, i.e., you slow down. Exactly why muscle force production declines is a difficult thing to pin down. Because there are so many things happening simultaneously when muscles are working hard, it is difficult to determine the exact cause of fatigue. Fatigue takes many different forms. Runners know that the fatigue associated with the 800 meters is not like the fatigue associated with the marathon. However, all causes of fatigue have one thing in common: They lead to a decrease in effective muscle force production.

Fatigue is not something specific to slow or average runners. Even world record holders fatigue; they just do it later in a run and from a much faster pace than the rest of us. Indeed, fatigue is necessary to protect our bodies from damage. However, the only way to get faster is to cause some damage so the fatigue occurs at a faster pace. To do that, you must repeatedly threaten the body's survival with training stimuli so that your body adapts and physiologically overcompensates. When the same stress is encountered again, it does not cause the same degree of physiological disruption. In other words, the body adapts to be able to handle the threat. 

Although some research supports that the central nervous system (CNS) can cause fatigue by reducing its command sent to the muscles–most notably experiments bypassing the CNS by electrically stimulating the nerve supplying the muscle of interest–the prevailing evidence and thought among scientists is that fatigue, at least during short-term exercise, is not caused by the CNS but rather by changes occurring in the muscle. The CNS seems to play a greater role in fatigue during prolonged exercise, like marathons and ultramarathons, when there are changes in the levels of the brain neurotransmitters, which increase the perception of effort and lead to feelings of tiredness and lethargy.

Since the CNS command to the muscles can be reduced from the CNS being inhibited directly from changes in brain neurotransmitters or from feedback from the muscles' metabolic condition, it's possible that under conditions that represent a risk to organs, especially the heart, inhibitory signals may be sent to the brain, an argument proposed by prolific researcher Timothy Noakes, M.D. (author of The Lore of Running). In response to these signals, the neural command is dampened and the muscles' power output consequently declines in an attempt to prevent damage to the heart. This may partially explain why your maximum heart rate is lower at altitude, since the lower oxygen availability would put the heart at risk at a lower heart rate than at sea level. Acknowledging the potential but unresolved role of the CNS, below are the main causes of fatigue for different races, with advice on how to combat them.