It’s running season again, and while I don’t recommend running for all (or even most) of my clients, I simply cannot live without it. The fascinating thing about running to me is testing the limits of my own fatigue. But this leads to a question: What is fatigue? What is the mechanism that allows you to only push yourself so far, and more importantly what is it about this mechanism that allows for the improvement of performance?

Quite simply, fatigue is an emotion. It’s an emotion that tells you increasingly and inevitably to slow down or stop moving outright. Emotions are incredibly deep, powerful, and ancient sensations of brain chemistry that arise from the oldest parts of the brain, the limbic system and midbrain. The limbic system is responsible for the way our bodies responds initially to stimuli. It predates our pre-frontal cortex (“PFC,” where all the real “thinking” occurs) and is responsible for the creation of emotions, which have great impact on the way our body responds to those stimuli. The emotion “fear” is really our brain telling preparing our body for “fight or flight.” The emotion of “bliss” is a release of brain chemicals that remind us we did something rewarding to our bodies or brain and to keep it up. These emotions are tied into the sense organs pretty directly and respond much faster than the PFC can even deduce what’s going on. If your friend sneaks up on you, you get startled and your heart races (a release of adrenaline coordinated by your amygdala and sympathetic nervous system) before you can even perceive cognitively whether there is a threat. And after your friend reveals himself, it takes a while to calm down.

The role of the pre-frontal cortex is cognition, your higher reasoning skills. It’s what you would consider, “you.” But interestingly, the main role of the PFC is to inhibit your emotional responses. In fact, your PFC produces most of your brain’s GABA, the neurochemical responsible for inhibiting synaptic firing. Without intervention from the PFC, your limbic system is a positive feedback loop. Your emotions just get more and more intense, like a dog gets more and more excited about going outside for his walk. Stopping this loop usually manifests itself as you telling yourself to “calm down” or “cheer up!” as you try and counteract stimuli coming in from the outside world. Fatigue is one of these deep-brain emotions, but the stimuli is internal.

Think about when you perceive a threat. You see it with your eyes, hear it with your ears, etc. These outside stimuli are translated into nerve signals by the sense organs and move into the limbic system, usually via the hypothalamus. But your body has a lot of senses, ones that you aren’t consciously aware of which face inward. Science has identified many of these senses as factors in what we experience as fatigue, or that urge to slow down. Here are the main ones that we know about.

  1. Lack of oxygen. After just a few minutes of anaerobic activity, your muscles need oxygen to keep moving. This is measured as total volume of oxygen your body can pump to your muscles in a given time and referred to as “VO2 MAX.” If you exceed your VO2 MAX for too long, your brain senses the lack of oxygen and fearing for your heart muscles (which cannot run anaerobicly) tells you to slow down.

  2. Depletion of fuel. Your body needs fuel to move the muscles and keep your brain thinking (your brain actually uses a lot of fuel) and it places a high priority on sugar. So as you burn up available ATP (2-6 seconds), then creatine tri-phosphate (6-30 seconds), then muscle glycogen (20 minutes), then the glycogen in your liver (45 minutes) your brain starts to get worried that it won’t have enough sugar. So it tells you to slow down.

  3. Acidity. The byproduct of a lot of the chemical reactions that produce muscular motion are H+ ions. If these ions accumulate faster than they can be shuttled into other area of the muscle fiber cells, the chemical reactions necessary for motion get less and less efficient. Scientists call this the “Lactic Threshold” followed by the “Onset of Blood Lactate” or OBLA, which is the actual point of diminishing marginal return in trained athletes. This is when the pH in your muscles and blood is high enough that your brain notices and tells you to slow down.

  4. Overheating. Moving your body produces heat and your body copes with that heat through cooling mechanisms like sweating. If you are producing more heat than your body can handle or if you have run out of water to sweat with, your brian will sense this and tell you to slow down.

  5. Muscle damage. When you are going all out, especially during ballistic activity like running where the muscle fibers are being stretched and shortened with many times the force of gravity, damage to the muscle fibers occur. These are repaired during recovery, but as the tears and tetanus accumulate faster than your body can deal with their waste products, and as you start to run out of other fibers to recruit (especially type 1 fibers) then your brain gets worried you are going to chew up all your muscle fibers and it tells you to slow down.

  6. Central nervous system neurochemical depletion. Your body needs nerves to signal the muscles to contract and relax in order to control movement. Synapses are gaps between nerve cells and muscle cells where chemicals are passed back and forth to create those signals. But there is a finite amount of those chemicals in your body, especially sodium. As you use them up, your brain gets worried you will use them all up and it won’t be able to function without sodium. So it tells you to slow down.

I’ve put these senses in order of how soon they appear in the feeling of fatigue to give you an idea of how long you have to work before you begin to feel that signal as a desire to stop. This model is called the “Central Governor Theory of Fatigue” and was posited by Tim Noakes. He outlines this theory in the must-read, The Lore of Running. The theory is really that the brain senses all these (and many other) stimuli and is constantly metering the body’s output, acting as a governor to your total performance. This means that fatigue is more lie an emotion (a subconscious response to stimuli through the limbic system) than a simple biological response. In short, fatigue is a complex system of inputs controlled by a very old part of our midbrain, not a simple negative feedback loop that looks at one thing and says, “not enough oxygen… stop!” Believe it or not, this is rather controversial and goes against 70+ years of assumptions about exercise science which to this day is obsessed with VO2 MAX.

So why is it that we can train our bodies and better our performance? How do we resist fatigue? In essence, we train our central governor to chill out. Our brains are very paranoid and there is a large envelope of performance that we are leaving untapped before we start to train. Training teaches our brains that we can still function at speed without catastrophic failure. there are biological responses too (like higher muscle glucose levels, later LT and OBLA, higher VO2 MAX and heart stroke volume, more type IIb muscle fibers, etc) but the major governor to our performance is our subconscious. Its that overwhelming desire to stop that our pre-frontal cortex must regulate as it would any other emotion that we determine to be unwanted or unreliable. We can do this by keeping our PFC engaged, which is fancy talk for keeping from getting bored. I do this with music and by concentrating on my form and breathing. If you are running splits, stay focused on your time. If you focus on emotion, you start to give in to it. And if you focus on fatigue, you will eventually just stop. So if you can keep thinking, you can keep moving.