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