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What is Muscle Fatigue and What Causes It?

The human body is a masterpiece of form and function, capable of doing amazing things in the physical realm. But when we push ourselves to the max, we’re all bound to hit a wall. Muscle fatigue is inevitable, whether we’re using resistance bands, engaging in aerobic activity, or simply going about our daily lives.

We’ve all experienced the sensations first-hand. Muscles start to weaken, pain sets in, and that burning sensation consumes all of your senses. In simple terms, muscle fatigue is our body’s way of saying STOP! But there is way more to this phenomenon than meets the eye. We have a complex set of signals and systems in our bodies that cause our muscles to tire and let us know if it’s safe to continue – or not.

Today, we’re taking a close look at what muscle fatigue really is and why it’s actually a crucial part of our physiology. We’ll examine the science behind muscle fatigue and explain the differences between “good” and “bad” pain that we may experience during our toughest workouts.

With this knowledge under your belt, you can safely push yourself to new limits, achieve goals you never thought possible, and get the fitness results you want most. Let’s go!

Exercise-Induced Muscle Fatigue

It’s not rocket science: the most common cause of muscle fatigue is overuse. To see this idea in action, step away from your device and try to do 50 consecutive bodyweight squats right now.

Seriously, get up and do the squats!

Even if you aren’t in fantastic shape, you’ll probably be able to perform 10 to 20 repetitions with no problem. Our legs are built strong to carry our weight, and it doesn’t cost us much energy to get those first reps out of the way.

However, once you start to reach that 30-to-40 range, you’ll find that things get tougher, no matter your level of physical fitness. Your heart rate will begin to rise, your legs might start to wobble, and you’ll notice that signature burning sensation in the areas doing the most work.

By the time you hit 50 squats (if you make it that far), we won’t blame you if you crash on the couch and need to catch your breath! Those legs will feel like they’re on fire, and you might need a glass of ice water to cool down.

That is muscle fatigue in action, and as you can see, it happens fast if the conditions are tough.

The same principles apply to movements of all types, whether you’re performing strenuous exercise or simply loading up your cart with supplies at the store. How much pressure and strain you put on the body will cause muscle fatigue to accelerate, burning more energy and forcing the cardiovascular system to work overtime.

Tiredness, exhaustion and discomfort can happen in the gym or daily life, and it’s due to the finite amount of energy we have in our muscles, among other factors.

Without getting too deep into the science, muscle force relies on tiny cellular engines throughout the body and requires neural activation. We ultimately control our level of output and the precise movements of our bodies.

Within each cell of the muscle, there is a respiration process similar to that of oxygen in our lungs. Whether it’s pushing, pulling, jumping, or stretching, every contraction makes those cells “breathe” and expend the energy contained inside.

Blood flow ensures that oxygen and nutrients make their way to the muscles, which is why our hearts begin to pump faster and breathing rate increases with heavy work.

The pain we experience comes from glycogen (energy) depletion in the muscles, and a buildup of metabolic byproducts that signals discomfort to the brain. This lets us know that the muscles are maxed out, and it’s time to rest and regain our energy once again.

When To Keep Going, When To Stop

All living things are averse to pain, and it’s what has kept us alive for thousands of years.

But the truth is that muscle fatigue is not going to put you in any real danger if you know what you’re doing. In fact, it will help you lose unwanted weight, build more muscle, and become mentally tougher for your next workout.

When muscles begin to tire out, that’s when you need to double down and stay focused. Like Muhammad Ali once said, he only counts the sit-ups when they begin to hurt!

This could mean pushing past the discomfort during a weightlifting session or a high-intensity group dance workout. It could also mean leaning deeper into a stretch or yoga pose that will release tension in the muscles and give you greater relief after the fact.

You’ve heard the expression “no pain, no gain,” – but it’s only true to a point. You should know when to back down so that you don’t sustain an injury since you need to stay in the game to hit your long-term goals.

For example, if you’re running or biking, take a breather if you start to experience dizziness or notice your balance start to slip. While using resistance bands, you should stop immediately if you feel any sudden or sharp pain in your muscles or joints.

Knowing the difference between good and bad pain is key to working out safely, and you’ll be able to find the sweet spot as you gain more experience over time.

Don’t neglect rest and recovery, either. In between workouts, you want to eat well, drink plenty of water, supplement with vitamins, and get deep sleep each night to help your body replenish all of those energy stores that got wiped clean.

That’s why so many top doctors simply recommend that you listen to your body when navigating your fitness journey. Chronic fatigue comes from overdoing it every day of the week without proper time for recovery, and the downsides just aren’t worth it.

Be aware of the signals you receive from your muscles and give yourself time to heal from high-intensity workouts. The best athletes in the world eat quality, clean food, sleep more than 10 hours a night, and do anti-inflammatory activities like saunas, ice baths, and deep tissue massages for this exact reason!

It’s a smart rule of thumb to follow: the harder and more frequently you train, the more consideration you should put towards recovery, nutrition, and holistic health. When you hit your next workout again, you’ll feel ready to take on the world.

Tips to Push Forward

It may not be what you want to hear, but muscle fatigue is actually vital to making progress in health, fitness, and any other goal, for that matter. It’s all about pushing yourself out of your comfort zone and discovering what’s possible on a mental and physical level.

Consider how athletes perform at the upper echelons of competition. In certain weight classes for fighting or weightlifting, physical capabilities are largely the same. Mental fortitude and resilience are what separates the best from the rest!

To achieve our dream physiques and level up overall health, we need to learn to deal with muscle fatigue and the mental strain that comes with it. When done safely, this is what builds muscle, burns fat, and upgrades our lives in all respects.

Of course, it can be challenging to self-motivate and push ourselves when nobody is around, which is why we have the Body FX team and our community to keep us accountable and on track with our routines.

You can leverage this team dynamic as well by joining our online fitness community that will support and coach you every step of the way. Muscle fatigue fades fast when you have the right people motivating and encouraging you each day!

While fitness is mostly a solo journey, we all need a boost now and then, and the right community can be an invaluable asset for anyone in the trenches.


Everyone experiences some degree of muscle fatigue, no matter our ability level, age, or chosen activity. It’s just a part of the game, and overcoming it requires a game plan involving nutrition, recovery, stretching, resting, and maintaining the right attitude.

When you know how to properly manage muscle fatigue, it becomes a useful compass that directs you towards bigger goals and better health. With experience, you will know when to push forward and when to pump the brakes.

Now that you know the truth about muscle fatigue, you can use these tips to your advantage and accelerate your fitness results safely and sustainably.