Here’s a thought experiment for you: A professor of exercise physiology at the local university asks you to come to his lab for an unspecified fitness test. He puts you on a treadmill, hooks you up to his sensors and gives you the following instructions: "You must run as fast as you can. The distance will be somewhere between 100 yards and 50 miles. Your performance will be rated against that of your peers. Ready, Go!"

Of course, you’re likely to protest such an unreasonable demand and the professor may have to offer some substantial incentives to get you to participate. But assuming that he succeeds, imagine how this challenge might unfold. How would you manage your body? How would you set a pace? You’ve got to run fast, but you’ve got to conserve resources. You’ve got to be powerful, but you’ve got to be efficient. It’s a serious conundrum, far more challenging than running a particular fixed distance.

As it turns out, this kind of fitness challenge has become a sort of Zen koan for a new generation of exercise scientists. A new body of research calls into question many of our assumptions about endurance, knowledge and the nature of fatigue. Believe it or not, fatigue is starting to become exciting.

Fuel Depletion and the "Final Lap Paradox"

This modern interest in fatigue science was featured prominently in the March 20, 2004 edition of New Scientist. In an article titled "Running on Empty," author Rick Lovett posed the question "Can it really be possible that fatigue is all in the mind?"

For years, the standard theory has held that muscular fatigue is caused by the build up of metabolic wastes like lactic acid, accompanied by the reduction in available fuel. This is the body-as-battery paradigm, otherwise known as the "limitations theory."

But this prevailing theory has now lost favor, due in part to the "final lap paradox." The problem is that if fuel depletion and waste accumulation were really the ultimate causes of fatigue, we’d expect to see runners start fast and then gradually slow down throughout a typical race. We’d see them behave like cordless appliances, progressively losing power over time. Instead, we see brisk, well-paced movement during the early going, followed by an impressive surge of speed and power on the final lap. This tells us that there must be something else going on here.

Consequently, exercise scientists are beginning to replace the "limitations theory" with a more sophisticated idea called "central governance." This new theory holds that fatigue is not caused by distress signals coming from exhausted muscles, but is rather an emotional response which begins in the brain. Contrary to perception, it’s not the muscles who are running the show. Instead, when the brain decides that it’s time to quit, it creates the distressing sensations that we interpret as muscle fatigue. The most surprising consequence of this line of thought is that fatigue may scarcely be a physiological phenomenon at all. Instead, fatigue may actually be a creation of the mind.

Interval Training

Support for the central governance theory comes from the success of athletes who practice interval training. As you may know, intervals are surges of effort that come in the midst of sustained exercise; typically, the athlete runs, swims or bikes at a moderate pace and then adds periodic bursts of speed. Experience shows that such training practices are remarkably effective in improving performance. The traditional explanation holds that such training improves oxygen uptake and the ability to tolerate metabolic waste. But increasingly, modern coaches say that the benefits of interval training are primarily, if not exclusively, mental. Experience with intervals tells the brain that "it’s safe to go faster." Once the brain becomes convinced that no harm will come to the body, it releases more resources, which in turn allows us to run, bike or swim harder than before.

Biochemistry

Scientists have even discovered a biochemical link in the process. Researchers from the University of Cape Town in South Africa have discovered a signaling molecule called interleukin-6 that appears to play a key role in creating the fatigue response. Blood levels of IL-6 are considerably higher than normal following prolonged exercise, and injecting healthy people with IL-6 makes them feel tired. To test their theory, researchers injected trained runners with either IL-6 or a placebo and recorded their times over 10 kilometers. A week later, the experiment was reversed. On average, the IL-6 group ran significantly slower.

The Power of Knowledge

We see examples of the central governor theory in almost every modern sporting event: the four minute mile, the 7 foot high jump, El Capitan in a day, deep water ocean swims, the 1,000 pound squat. All of these feats have been made possible, not so much by fitness, but by knowledge. Today’s high-knowledge athletes aren’t really that much "fitter" than their counterparts of the past, but they have immensely more knowledge about what they’re doing. This allows them to lower the setting on their central governors.

When Sir Edmund Hillary first roped up at the base of the Khumbu icefall in his attempt on Mt. Everest, we can be sure that his brain was mightily impressed with the extremity of what he was about to do. Unfamiliar terrain and unknown demands loomed above him. His brain had no prior knowledge of this environment and no idea what to expect. Consequently, we can be sure that Hillary and his party experienced major fatigue almost as soon as they left base camp. That’s one reason why they needed a series of camps up the mountain. The lack of oxygen was a challenge to be sure, but it was the lack of knowledge that really slowed their pace.

In contrast, the modern climber comes to the Himalayas armed with detailed knowledge. Mt. Everest has been mapped down to the last crevasse and even armchair mountaineers have some idea of the challenges involved. And so, today’s mountaineer races up the slope, his brain comfortable in the knowledge that, however life-threatening this environment might be, it is still within the realm of the known and the possible. The body of the modern mountaineer may be somewhat fitter than Edmund Hillary’s, but his brain is miles ahead. And so, today’s Everest climbers manage the feat with remarkably little fatigue.

Life on the Grassland

Judging from the way it creates fatigue and limits physical exertion, we can conclude that the brain’s default strategy is fundamentally conservative. In effect, the brain seems to be programmed with this simple rule: "In unknown circumstances, create fatigue to limit exposure and vulnerability."

This strategy makes good evolutionary sense. After all, the human brain evolved in a hostile environment where staying alive was the first priority. If physiology was allowed to simply run amok, the organism would be exposed to excessive risk; there’s no telling what might happen.

Consequently, evolution has supplied us with a governor, a neurological brake on physical exertion. In this sense, fatigue is a dampener, a restriction device that protects the organism from excessive activity. Natural selection would favor individuals with such an internal program. Individuals with a healthy fatigue-creation capability would be more likely to survive than fatigue-deficient counterparts.

The Unknown Distance Run

Our new understanding of fatigue also leads us to the inescapable conclusion that, because of their predictability, modern athletic events are completely artificial. If your event is the mile run, you know your distance down to the inch. You know that you’ll be running four laps, no more, no less. You know that the running surface will be smooth and regular. There will be no obstacles and no surprises. The challenge will be utterly predictable and your knowledge level will be extremely high. Such high levels of predictability occur nowhere else in human history or in nature.

From an evolutionary point of view, a more realistic athletic event is the military’s "unknown distance run." In this event, participants race against one another, but no one in the contest knows how far they’re going. This makes sense in terms of military training because in combat, you never really know how long you might have to go to reach safety.
It also makes sense in terms of human history because on the grassland, anything can happen. In primal human environments, physical challenges are open-ended. When hunting, you don’t know how far you’ll have to walk before you score a kill. When being hunted, you have no idea which animals will chase you or for how long. There are lots of unknowns out there.

Into the Unknown

So, the question now becomes "How can we use this new understanding of fatigue to improve our personal health and fitness? Should we seek out high-knowledge environments like sporting events or should we do our training in situations of physical mystery? What’s better for your body? Perfecting your ability to perform a single, familiar movement or subjecting your body to unknown physical challenges?

Some facts are well known. When we start a training or conditioning program, almost any kind of physical challenge will give us improvements in health and fitness. As we struggle to move our bodies in some particular way, we sweat and strive and our bodies become stronger and fitter. As we run or bike or lift, we gain fitness and knowledge simultaneously.

But over time, our increased familiarity leads to a stale, less responsive physiology. If all you ever do is run the mile, your physiology will make specific adaptations to support your performance in this event. In the beginning, the challenge will be highly stimulating to your body, but as conditions become increasingly familiar, your body will begin to slack off. In this way, familiarity becomes an enemy of conditioning. The more you know, the less you grow.

Body builders and weight lifters are very familiar with this process. Start a program from scratch and you’ll make solid gains over the first few weeks. Your nervous system will get smarter and faster and you’ll go up in your weights. Your encounter with physical novelty will stimulate actual physical transformation. You’ll be adding weight to the bar almost every session.

However, if you stay with the familiar program of sets and reps, you’ll soon hit a plateau. You won’t be adding weight to the bar anymore and you’ll start to get frustrated. This is a familiar, well-documented pattern in gyms around the world. Trainers attempt to explain this phenomenon in physiological terms, but it may just be psychological. The reason the body stops adapting is because the challenge has become too familiar. As knowledge increases, the body begins to coast.

That’s why trainers advise people to mix up their programs every six weeks or so. Introduce a fresh challenge by changing the exercises, the sets or the reps. This adds welcome diversity, but it also decreases your knowledge. You don’t know what it feels like to do this new pattern and so you have to dig deeper into your physiology. As Arnold Schwarzenegger put it in Pumping Iron, "You must surprise the muscles."

This is good advice, not just for muscle tissue, but for all the systems of the body. Surprise your muscles, surprise your nervous system, your heart, your lungs, your sensations and your expectations. The key to sustainable, long-term physical fitness success lies in our relationship with novelty and our willingness to expose ourselves to new challenges. The less we know, the more we'll grow.

So plunge yourself into some low-knowledge situations and let your body fight its way out. At first, the challenge will be difficult and exhausting. But as knowledge and familiarity grow, your fatigue will shrink and your competence will expand.