What Is Fatigue? Muscles, Willpower, and Mental Toughness

What is fatigue?

Fatigue is a state of weakness.

Muscle fatigue is the temporary physical inability to use one’s muscles optimally.

Willpower fatigue is the temporary mental inability to self-regulate (control oneself) optimally.

The Strength Model

The strength model of willpower fatigue claims that willpower relies on a common and limited energy source that can be temporarily depleted. According to the glucose depletion hypothesis, this energy source is glucose.

The depletion of glucose/willpower is called ego depletion. For example, if you try to not get distracted, you’ll have less willpower to resist cookies afterwards; or if you suppress your emotions, you’ll have less willpower to persist at a physical exercise afterwards.

This model has several problems:

Willpower performance increases consequent upon sleep without a concomitant increase in glucose levels.
Any single willpower task only expends a negligible amount of total brain energy and is unlikely to noticeably decrease blood glucose.
Brain glycogen levels aren’t responsible for the termination of even high-intensity physical exercise.
If you rinse your mouth with a glucose solution and then spit it out, your willpower will get less fatigued, even though the uningested glucose doesn’t affect blood glucose.
Vigorous physical exercise decreases blood glucose but improves performance on some complex mental tasks.
Blood glucose (unlike willpower exertion) has domain-specific effects; for example, its influence on decision-making depends on whether the decision is relevant to acquiring food or not (Orquin & Kurzban 2016).
Motivation can override the ego depletion effect (which may not even exist).

The Motivational Model

According to the motivational model of willpower fatigue, willpower isn’t limited. Using willpower simply triggers a shift in motivation and attention. Willpower fatigue just means that you’re less motivated to control yourself. Conversely, motivation can override willpower fatigue. If you get motivated, better yet, motivate yourself, if you stop worrying about energy conservation, and if you make autonomous choices, your willpower won’t get fatigued.

However, this model has problems too:

It neglects the importance of homeostatic mechanisms and physiological substrates.
It can’t explain why motivational shifts occur or why willpower gets fatigued.

Central Governor Theory

The central governor theory is a theoretical framework to explain muscle fatigue, but it can also provide an integrative account of willpower fatigue without creating the problems of the other models.

Traditionally, metabolic muscle fatigue was thought to reflect a “catastrophic failure” of homeostasis due to a depletion of substrates (glycogen, creatine phosphate, and adenosine triphosphate), an accumulation of metabolites (waste products), or both.

As we know today, these mechanisms don’t directly cause muscle fatigue. When people terminate an exercise because they feel too exhausted, those substrates are usually far from depleted.

What typically limits people’s physical performance is central fatigue, which is a fatigue based on chemical changes within the central nervous system (= brain + spinal cord). Well before muscle energy reserves are spent, central fatigue limits the conscious recruitment of motor neurons and muscle fibers. Not your muscles but your brain is responsible for the fact that you typically can’t express maximal voluntary strength.

This explains a lot of things, for example:

Hypnosis, music, deception regarding workload, certain beliefs, etc. can manipulate muscle fatigue. Why? Because it’s central fatigue.
Rinsing your mouth with a glucose solution without ingestion can manipulate muscle fatigue, but intravenous infusion of glucose cannot. Why? Because it’s central fatigue.

The central governor is the mechanism underlying central fatigue. It limits physical exertion to prevent homeostatic breakdown and causes subjective feelings of fatigue. You stop, when your central governor tells you to stop; not when your muscles fail because that’s exactly what the central governor tries to prevent—it protects you from tearing muscles and rupturing tendons.

Still, if you’re extremely motivated, you can override the central governor’s normal limits. That’s mental toughness. Mental toughness means that you can, with the power of your mind (or, as I argue, with the power of your pride), create enough motivation to override the normal limits of the central governor; for example, during high-stakes sports competition and extreme endurance events.

Also, when your life is threatened, these limits can be modified, for example, in war. Or have you ever heard of those miraculous stories in which normal people suddenly gain superhuman strength enabling them to lift cars to free pinned accident victims? Some of these stories are documented and they’re instances of hysterical strength, which is, again, based on an overriding of the central governor’s normal limits.

This picture depicts the mechanism and inputs of the central governor:

To learn more about the factors that inform and affect the central governor, watch this video:

You can strengthen your willpower by manipulating some of these factors. For example, if you learn to value more the importance of an activity, or willpower exertion in general, then the psychological benefits will increase, which will elevate your motivation, override the central governor, and thus prevent willpower fatigue.

Furthermore, the central governor model implies that you can overcome willpower fatigue under virtually any circumstances—if you have enough motivation, particularly intrinsic motivation (the effect of extrinsic motivation is generally low, which is, by the way, why most laboratory studies on ego depletion are worthless).

Muscle Fatigue = Willpower Fatigue

This theory also suggests that muscle fatigue and willpower fatigue are largely the same phenomenon:

Structurally, both share the central governor framework.
Neurally, both are associated with similar brain structures; for example, prefrontal cortex (Tanaka & Watanabe 2012), anterior cingulate cortex (ibid.), and anterior insula (Tops et al. 2013).
Chemically, both have the same physiological inputs influencing the experience; for example, stimulants and carbohydrates increase, while cytokines (proteins built by the immune system to trigger “sickness behavior”) decrease, both muscle and willpower performance.
Behaviorally, both are associated with an impaired ability to perform optimally at a challenging task.
Experientially, both are associated with low desire to perform optimally at a challenging task.
Psychologically, both can be affected by motivation, beliefs, and other cognitive processes.

However, one difference seems to be that, in addition to the central fatigue, there may be some local physical limitations of how much force a muscle can generate. While willpower fatigue is based solely on central fatigue, muscle fatigue might additionally depend on an exercise-induced accumulation of metabolites within the muscle fiber.

Why do we get fatigued?

Similar to pain, muscle fatigue alerts you when your body conditions are no longer optimal. It motivates you to rest and refuel in order to protect the homeostasis of the body and prevent physical injury.

Willpower fatigue has probably co-opted the pre-existing neural mechanisms of muscle fatigue for homeostatic maintenance in order to conserve energy.

More importantly, however, willpower fatigue limits behavioral flexibility. Having willpower and being flexible in your behavior is great:

When there’s a lot of sugar to eat, you control yourself, close the box of donuts, and don’t get fat.
When there’s a lot of stimulation to consume, you control yourself, turn off the TV, and go move your body.
When there’s a lot of social conditioning to follow, you control yourself, overcome your fears, and hit on sexy ladies.

However, when you control your hunger, attention, or fears for too long, you’re likely to starve, overlook a real threat, or blindly run into death because you don’t give a fuck. Your basic instincts, drives, and emotions are there for a reason: to survive. Especially in the modern world, being able to control your impulses is beneficial, but if you could do it all the time with unabated willpower, you just wouldn’t survive.

It’s awesome that you can suppress your sex drive when your best friend’s wife makes you horny or when the Internet allures you with porn, but if you suppressed your desire for sex constantly, you’d fail to reproduce.

Simply put, willpower fatigue ensures that you consciously control yourself episodically, not chronically. It serves to maintain a balance between conscious mental power and visceral body intelligence—you need both.