Have you ever wondered why your legs feel heavy when you exert yourself while your heart rate and your breathing aren’t going up? If muscular fatigue limits you in your physical activities, you certainly have a muscular endurance deficit which prevents you from expressing your true level and this, whether you practice CrossFitⓇ, cycling or running.

In this article I’m going to explain the physiology of fatigue and then the role of slow fibres and fast fibres during an endurance effort. I’ll also show you which exercises you can use to condition these fibres effectively and deal with muscle fatigue in the legs to increase your performance.

But first, we’ll look at exactly what causes muscle fatigue (and no, it’s not lactate or even lactic acid). If you’re new to the blog, I’m Sean and I help CrossFitⓇ athletes train harder and perform better through better endurance.

Muscle fatigue: The lactate (or lactic acid) myth

It may surprise you, but the reason we still hear that lactate causes fatigue dates back almost 200 years. Lactate was first measured in humans in 1843 by a German physicist and chemist named Johann Joseph Scherer.

In simple terms, lactate is a half sugar. When you break a glucose molecule in 2 to recycle energy in the body, you end up with 2 lactate molecules.

A few years after the discovery of lactate in human blood, lactate was also found in significant quantities in the blood of deer after exhaustion hunting sessions.

As the deer were completely exhausted and their lactate concentration was high, the researchers concluded that lactate was the cause of muscle fatigue.

And this myth is still present in 2024.

But if it’s not lactate, what really causes muscle fatigue?

Real Causes of Muscle Fatigue (in Sport)

If we look at this 2022 study, we learn that muscle fatigue is mainly caused by 2 molecules. One is responsible for what is known as peripheral fatigue (in the legs) and the other for central fatigue.

In the first case, it’s a molecule called inorganic phosphate (abbreviated Pi) which increases in concentration in the muscle fibre up to a certain threshold, beyond which the contractions of this fibre are weakened.

A bit like when you fill up your bath. As long as the water level doesn’t exceed the height of your bathtub, no worries. But as soon as the maximum threshold is exceeded, the whole house becomes a mess.

In the second case, it’s the hydrogen ions, abbreviated H+, that seem to affect the afferent nerves, i.e. the nerves that communicate with your brain. This generates central fatigue which then prevents you from contracting your muscles as hard as before, no matter how hard you try.

Inorganic phosphate and hydrogen ions are both by-products of our metabolism. And to put it simply, the greater the intensity of your effort, the more of these metabolites you produce.

So to sum it up, muscle fatigue is not caused by lactate or lactic acid, but rather by the accumulation of inorganic phosphate in the muscle and the general accumulation of hydrogen ions in the body.

So what can we do to better recycle these metabolites and delay muscle fatigue?

Understanding Muscle Fibres and their Role in Muscle Fatigue

To answer this question, we first need to understand how our different muscle fibres work during exercise.

Simply put, each muscle is made up of two types of muscle fibre, called slow-twitch fibres (type 1) and fast-twitch fibres (type 2). Their name comes from their contraction speed. Indeed, the force produced by slow and fast-twtich fibres is identical.

But because fast-twitch fibres contract much faster, they produce between 5 and 8 times more power comparend to slow-twitch fibres. So fast-twitch fibres produce more power than slow-twitch fibres. And that’s good, because those Type 2 only recruited when a large amount of force needs to be produced.

Basically, we can say that at low intensity, your body recruits mainly slow-twitch fibres to produce the effort required. And as the intensity of the effort goes up, you’ll recruit more fast-twitch fibres to cope with the increasing demand.

So what’s the link between the different fibres and muscle fatigue?

Mitochondria: The Cellular Recycling Plant

You now know that slow-twitch and fast-twitch fibres contract at different speeds. But that’s not the only difference between these two types of muscle fibres. Inside each muscle fibre is a network of mitochondria. These descendants of bacteria literally allow you to breathe, as it is within the mitochondria themselves that your body uses the oxygen you absorb.

It’s also in the mitochondria that you recycle a large proportion of the metabolites produced during exercise. And we know that almost 3/4 of inorganic phosphate is recycled directly in the mitochondria.

It turns out that slow-twitch fibres have around 3x as many mitochondria as fast-twitch fibres. They are therefore able to use much more oxygen and recycle much more Pi. What’s more, fast-twitch fibres produce far more metabolites than slow-twitch fibres. You could say that slow-twitch fibres are more efficient than fast-twitch fibres.

So, to sum up, fast-twitch fibres are very powerful but they also produce a lot of metabolites which then have to be recycled by the mitochondria inside the slow-twitch fibres. Otherwise, these metabolites accumulate and lead to muscle fatigue (in the legs or elsewhere).

With this information, we can determine exactly what type of training will enable you to improve your muscular endurance and cope with increasing muscle fatigue.

The training intensity that limits muscle fatigue

To know how to train your muscles to better recycle metabolites such as Pi and H+ and cope with increasing muscular fatigue, you need to understand a fundamental principle of sports preparation called specificity.

The principle of specificity tells us that the body always adapts in a specific way to the stress imposed on it. For example, if you train for several weeks at altitude, the lack of oxygen will force your body to generate more red blood cells to better transport the little oxygen available.

So to develop the capacity of our slow fibres to recycle the metabolites produced by our fast fibres, we need to use training that imposes this specific stress on our body.

Contrary to what some people say, it’s not at high intensity that you need to train to generate this type of adaptation. Because at high intensity, you produce far more metabolites than you can recycle, which means you’ll quickly tire and have to slow down or even stop your effort.

So you need to find an intensity that allows you to maintain your internal balance (called homeostasis) in order to prolong the stimulus and give your fibres time to adapt.

We’re talking about medium intensity here.

The right Training for Muscular Endurance

Medium intensity exists between the two physiological thresholds. If we talk about zones, we commonly call this type of training tempo (Z3) or threshold (Z4) training.

In running, the pace of these sessions corresponds roughly to that of a half-marathon or 10km. For cycling, you can think of an intensity that can be maintained for between 1 and 3 hours.

These sessions will be the most beneficial and will enable you to drastically increase your muscular endurance and cope with the accumulation of fatigue.

But given their relatively low intensity, are these sessions still useful for high-intensity sports like CrossFitⓇ?

Muscular Endurance for Intensive Sports

Here too, we often hear that if we want to train effectively, we should only perform activities that resemble our sport.

And that’s the biggest lie in the world of sports performance. In fact, I consider these medium-intensity sessions to be extremely useful and even essential for short, intense sports.

It’s these sessions that will help you train more often, recover better, lower your heart rate during exercise and maintain high intensities for longer.

It’s exactly this kind of session that I use in my Bike Endure programme, which to this day has helped hundreds of CrossFitⓇ athletes, cyclists and other sports enthusiasts to increase their muscular endurance singificantly, cope with fatigue and improve their recovery during and after each training session.

Now that you know how to improve your muscular endurance with medium-intensity training, all you have to do is include these sessions in your programme for 8 to 12 weeks to see your performance take off!

And if you’re looking for a complete programme that will enable you to improve your FTP test by an average of 13% and lower your heart rate in zone 2 by 8 beats per minute, click on this link to join over 400 athletes who have already taken part in the project!

Thank you for reading this article, I look forward to your questions in the comments section. See you soon!