What Causes Fatigue During Exercise?

What Causes Fatigue During Exercise?

You’re here because you want to know what causes fatigue during exercise.

You might be asking about regular tiredness during a workout – what are the mechanisms that cause you to feel tired while you’re working out?

Or you could be asking because you feel like the fatigue you feel during a workout isn’t normal, and you want to know why you’re feeling so exhausted.

more: The 9 Biggest Reasons You’re Tired All The Time

Whatever the reason, you want to know why the body begins to feel fatigued during exercise.

Here’s the answers:

A Combination Of Changes In Your Body Causes Fatigue During Exercise

Fatigue during exercise is caused by a few factors related to changes in your body. Lower levels of creatine phosphate in your body, changes to the serotonin levels in your brain, changes to your hormonal levels (especially cortisol), a rising core temperature in your body, dehydration and its effects, and lower levels of glycogen all deplete your energy and cause you to feel fatigue. If you’re feeling chronic fatigue for a period longer than 24-48 hours after a workout, talk to a doctor about your symptoms.

 causes of fatigue during exercise

more: The Biggest Reasons You Feel Tired During Your Workouts

Lower Levels Of Creatine Phosphate

Your body uses a molecule called atp as fuel – essentially atp is what your body uses to contract your muscles.

Muscle contraction is at the root of all movement – and that means during a workout your body needs to generate a lot of atp.

more: Diabetes Fatigue Might Be Why You’re Tired All The Time

Creatine phosphate’s role in your body is to keep your levels of atp above a threshold – so that you have the energy to move around.

After exercising for a while, the level of creatine phosphate in your body declines, leading to less atp production and causing you to feel more fatigued.

Increase To Your Cortisol Levels

Cortisol is an extremely important hormone in your body, sometimes called the “stress” hormone. It works as a proinflammatory and governs many aspects of your body.

more: The Top 15 Reasons You’re Always Tired

When you start exercising, normally the level of cortisol in your body actually goes down.

But as you continue exercising, that level goes back up, above the normal resting level and into higher concentrations. That is a factor that contributes to fatigue during exercise.


Dehydration is when your body doesn’t have enough water to continue functioning at peak performance.

We’re all familiar with the symptoms of dehydration, as pretty much everyone has experienced it at one point or another.

When you exercise, you sweat from the exertion, which accelerates dehydration and creates a greater need for fluids.

If you don’t replace your body’s fluids during exercise, dehydration definitely contributes towards fatigue.

Serotonin Levels Rise

Serotonin is a neurotransmitter in your brain. Increased levels of serotonin lead to feelings of sluggishness and tiredness.

When you exercise for a while, the levels of serotonin in your brain go up. Why?

more: The Science Of Muscle Fatigue

The reason is that your muscles actually release something called tryptophan while you work out. Tryptophan increases the level of serotonin production in your brain, which could lead to fatigue.

Loss Of Glycogen

Glycogen is a molecule used by your body to store energy. Your muscles use glycogen to store energy from carbohydrates.

more: 9 Strange Reasons You’re Tired All The Time

When you’re exercising, your muscles are activating that stored glycogen and using it up.

So when your muscles run out of glycogen, you start to feel the effects of fatigue and tiredness. This could be considered the main cause of fatigue.

more: What Causes Fatigue In High Intensity Endurance Sports?

Your Body Temperature Goes Up

As your body temperature rises, it puts more stress on your cardiovascular system to keep up.

Your body kicks in with cooling measures to try to cool itself off – which means you start sweating – and you lose more fluids that way.

more: Fending Off Adrenal Fatigue

In addition, your muscles are under more stress from higher temperatures, which all contributes to feelings of fatigue.

30 comments… add one
  • olivia May 16, 2018, 5:40 pm

    i’m planning on going to the gym this summer and it’s just awesome that i came across this post. i should really look out for these things to make sure i’m doing it right.

  • nerissa May 17, 2018, 7:07 pm

    i easily get tired whenever i do exercise and it is not a good thing. i always wonder what could be the reason for that. this article provided me the answers i was looking for.

  • bree May 18, 2018, 7:26 pm

    this is a good read and i might as well share it to my friends. i sometimes feel tired easily during exercise and i might consider these.

  • pauline May 21, 2018, 7:56 pm

    i was searching for mechanisms that causes me to feel tired while i am working out and this is the first article that popped up. it is very detailed and easy to understand. thanks for this wonderful post.

  • lucia May 22, 2018, 4:07 pm

    look out for dehydration! this is very common but too often neglected. always bring a bottle of water with you esp now that summer is almost here.

  • maddie May 24, 2018, 2:24 am

    i know the importance of exercise so i make sure to incorporate it into my lifestyle. sometimes i notice that the more i exercise, the less energy i have. i will see my doctor and talk to him about my symptoms.

  • celine May 24, 2018, 10:56 pm

    this is something i never saw coming. well, not that i don’t get tired when i work out..it’s just that i thought it’s normal to feel tired. i might as well consider these and take a deeper look.
    at my health.

  • iris May 25, 2018, 6:20 pm

    my workouts are tiring but i always get through them. for me, feeling tired during a workout is normal as long as it is not longer than 24 hrs. 🙂

  • reina May 29, 2018, 8:35 pm

    exercising or not, you need to hydrate. water cures everything.

  • cassey May 30, 2018, 5:52 pm

    this is a very detailed article. i was wondering why i always feel tired during my workouts and this post clearly explained everything.

  • sylvia May 31, 2018, 7:10 pm

    i always make sure that my body has enough water during a workout so i could continue functioning at top performance. and yes, i don’t feel tired whenever i am hydrated.

  • genesis June 4, 2018, 9:13 pm

    i am guilty of dehydration… this is the main reason why i always feel sluggish during my workout and i am doing something about it now.

  • donatella June 6, 2018, 8:12 pm

    i want to fight workout fatigue forever! will take note of what i’ve learned from this article and make sure that i am doing everything right. thanks for sharing this post with us.

  • gloria June 7, 2018, 7:42 pm

    hmm, so that what happens when our muscles run out of glycogen. this is a good read. I really learned a lot.

  • audre June 8, 2018, 8:26 pm

    i always wonder why i always feel fatigued during exercise. now i know what could be the reason is. of course, i will still consult my doctor bout this.

  • shirl June 11, 2018, 8:57 pm

    it is good to find out the biggest causes of being too tired to exercise and how to fix it. will help a lot of women like me.

  • pamela June 12, 2018, 9:12 pm

    i always feel tired even when i workout. it just doesn’t make any sense. good thing i read this post. i understand the possible causes clearly now.

  • carol June 14, 2018, 3:17 am

    now i know the factors that contribute to fatigue during exercise. this is a great article that active women like me must read.

  • susie June 15, 2018, 3:52 am

    i love working out in an airconditioned room and I don’t get tired easily.

  • priscilla June 16, 2018, 5:03 am

    lately, my workout feels harder and i don’t have any idea why. i will read this post carefully, maybe the reason is discussed here.

  • letty June 19, 2018, 8:23 am

    regular tiredness during workout is not normal for me. i love working out and never experienced any of these. Thank goodness

  • zandra June 20, 2018, 2:26 am

    i really need to have healthy lifestyle changes! thank you so much for this wonderful post. this will help and keep me from hitting a wall in my workout!

  • genevieve June 21, 2018, 3:48 am

    i have adrenal fatigue and unfortunately, an excessive or incorrect exercise can be harmful for me. it can also become one of the causes of why i am feeling fatigued because of my condition.

  • sicily June 22, 2018, 3:34 am

    it’s nice reading this post although i’ve never experienced being tired while working out. it feels great to learn new things especially health-related topics that i can use in the future.

  • emilly June 22, 2018, 6:29 pm

    i feel very lucky i came upon this post! i am thinking of going to the gym and what i’ve learned from this article can really help me! thank you so much!

  • alina June 27, 2018, 3:17 am

    excellent article! you clearly explained the causes of fatigue during exercise. i always end up to your site if i want a quick answer about health-related concerns.

  • joyanne July 3, 2018, 5:23 am

    we just need to find the right balance of nutritious food and sleep to get the energy we need to be able to workout. water is an important ingredient too!

  • leila July 4, 2018, 1:53 am

    well, this one is a great article and gave me a better awareness of why our bodies get tired during workouts. i always want to skip exercise because of fatigue. now i know how to manage it. thanks.

  • lina July 4, 2018, 10:32 pm

    water is life. we need it to eliminate toxins, maintain healthy skin and hair and also to avoid fatigue. we should be aware of the importance of proper hydration.

  • rose July 6, 2018, 4:17 am

    my husband and i have plans of going back to the gym and it is a perfect timing that i came across this post. i learned a lot, especially what causes our body to get tired while working out.

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