Is fatigue during exercise caused by tired muscles? Or, could it actually have more to do with your brain than your body?
In the 19th century, physiologist Angelo Mosso proposed the notion that fatigue had little to do with your body and nearly everything to do with your mind. (1)
At first blush, this idea seems ridiculous. Anyone who has exercised to the point of exhaustion would argue that the muscles physically fatigue and cannot contract any longer. Those of you suffering from adrenal fatigue, CFS, or any other condition where fatigue plays a prominent role will also likely be rolling your eyes. And rightly so.
Could fatigue really just be in your head?
What exactly is fatigue?
Fatigue can be manifested as difficulty or inability initiating activity (perception of generalized weakness); reduced capacity maintaining activity (easy fatigability); and difficulty with concentration, memory, and emotional stability (mental fatigue). (2)
A 2-week cross-sectional survey done in the United States found that 38% of people suffered from fatigue. (3) To be clear, this number represents pathological forms of fatigue. If we were to include people who also suffer from fatigue during their workouts, the number would likely be near 100%.
This post is related to fatigue experienced during exercise. This is an entirely different type of fatigue than chronic fatigue syndrome (CFS) or adrenal fatigue. Both CFS and adrenal fatigue describe underlying physiological processes that cause fatigue.
In this post, I focus on how fatigue during exercise could actually be a brain-derived condition and have little to do with your muscles. For those readers with CFS and/or adrenal fatigue, you may find some of the below content very interesting. I’d encourage you to read on!
What causes exercise fatigue?
Fatigue can be the result of a chronic illness – it is exhausting being sick for a long time. Fatigue can be a side effect of a medication. Or, fatigue can be the illness unto itself – like in conditions such as adrenal fatigue, hypothyroidism, iron deficiency anemia, or chronic fatigue syndrome. Exploring all the potential causes of fatigue is beyond the scope of this post. Instead, I want to focus on fatigue framed through the lens of your brain during exercise. This is called the central governor theory.
The central governor theory suggests that your brain regulates your exercise intensity to a level that is thought to be safe for your body. (4) This means that it’s your brain that stops you from pushing through to do an extra rep on your back squats or to run an extra mile. It is thought that your brain tries to keep the intensity of physical activity (exercise) tightly controlled so that the exercise does not threaten your body’s homeostasis.
Have you experienced the feeling of not being able to do any more reps at the gym?
That’s the central governor at work. Your brain stops you from doing one more rep by reducing the recruitment of muscle fibers in a given muscle group. It’s this decrease in muscle fiber recruitment that causes you to experience fatigue in your muscles. But it’s not your muscles that are tired. It’s your brain’s effort to protect your muscles and body from further damage.
The central governor theory was suggested to help explain fatigue after prolonged strenuous exercise in long-distance running and other endurance sports by Tim Noakes in 1997. (5)
Your brain on exercise
What does your brain do while you’re exercising?
That’s the area of study Tim Noakes specialized in. In Tim’s studies, it is thought that your brain is continually monitoring the power output of muscle groups. If your brain notices too much power going to a specific group of muscles, it will decrease the number of fibers firing in that group. You’ll experience the decreased firing of fibers as muscular fatigue.
If you’re doing lunges, you’ll (likely) start to feel fatigue in your quadriceps and gluteal muscles. Soon, you’ll need to either stop the movement or decrease the weight. If you’re running, you may feel the fatigue in your calves, hamstrings, quadriceps, and/ or glutes; your running pace will start to slow.
Your brain is often preparing you for a worst-case scenario. Think back to the days of your hunter-gatherer ancestors. During these times, it would be foolish to use up all of your energy reserves doing any one activity. If you did, you’d have no energy left to deal with sudden threats. In this era, the threats were more likely to be life-threatening – lion/tiger/bear encounters, invasion of a warring tribe, food shortages, etc. If you used up all of your energy, you would have likely died.
It is this same cautious approach that your brain uses today. Even though you (hopefully) no longer have life-threatening situations to deal with, your brain still wants you to keep some energy in reserve. (6, 7) In order to do this, your brain makes you experience fatigue. When you feel tired, you’re more likely to stop. All of this happens outside of your conscious control.
What (really) causes your muscles to fatigue?
Don’t your muscles fatigue because they’re lacking oxygen and/or glucose?
Yes. And no. Here’s why:
When you exercise, the mitochondria in your muscles cells need to create ATP. Think of ATP as energy for your cells. In order to do this, your mitochondria take glucose from your blood go through a complex chemical reaction called the Krebs cycle. The series of chemical reactions in the Krebs cycle creates ATP, or, energy for your muscle cells to use; which in turn allows you to continue exercising.
ATP is needed for all muscular movement. If you’re exercising at a rapid rate, your body may not be able to produce enough ATP (energy) to keep up with the demands of the workout. If your body struggles to keep up with ATP production, lactic acid will accumulate in your muscle tissue. Too much lactic acid build up in muscle results in muscular fatigue – or, your inability to do another rep.
If you want to continue exercising, you’ll need to slow your pace or take a short break. This will allow the lactic acid in your muscles to be replaced with ATP. Which gives your muscles the energy they need to continue exercising.
Contrary to popular belief, it is not lactic acid that causes soreness in your muscles. By the time your breathing rate has returned to normal, your cells have converted lactic acid back into glucose through another part of the Krebs cycle. The soreness you experience after exercise is due to tiny tears in your muscle fibers.
The healing of these tears causes an increase in muscle size as well as the number of mitochondria found with the tissue. This should (in theory) allow your muscle(s) to workout harder and longer after they have healed. Remember to allow your muscles time to recover after exercise. They need rest in order to grow new mitochondria.
How your brain intervenes during exercise
Now, you’re familiar with the detailed intricacies that go on in your muscle cells during exercise.
So, how does your brain meddle with this process?
Do you remember earlier in the post when I talked about how your brain is able to decrease the number of muscle fibers your body can recruit? Let’s imagine a simple example where you have 100 muscle fibers in your biceps muscles. You’ve done 10 bicep curls at the gym of a challenging weight. You’re utilizing all 100 of your muscles fibers for the first ten reps. But now, you start to notice slight fatigue in the muscles.
This fatigue is your brain signaling to decrease the number of muscle fibers firing in your biceps. Instead of 100 fibers, now your brain has reduced your capacity to 75 (these are arbitrary numbers used solely to better illustrate this example). At 75 fibers, you have 25% fewer mitochondria helping energize your muscle cells.
What happens next?
The next few reps become very challenging. In response, your brain decreases the number of fibers firing even more. Now, you’re down to 50% of mitochondria and muscle fibers. At this point, you’ll likely only be able to squeeze out one or two more reps. Then, it’s time to rest.
Your brain interjects your workout to ensure you:
a) Save some energy for later
b) Don’t do permanent damage to your muscle(s)
In all likelihood, your body is more than capable to exercise much longer and more intensely that you believe. Many high-performance athletes have trained themselves to push through the muscle fatigue and discomfort in order to achieve amazing feats.
For the rest of us, whose ambitions are more aligned with health and wellbeing as opposed to the highest performance, try leaning into the discomfort just a little longer than you’re used to! Instead of stopping when you start to experience muscle fatigue and/or soreness, see if you can stay with that discomfort for just ten more seconds. This is how you’ll continue to experience improvement in your given exercise pursuit(s).
Remember, don’t push it for too long. And always ensure you’re taking recovery days. Overtraining can be a sneaky cause of fatigue.
