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.
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.