Muscle Fatigue: Causes, Symptoms & Treatment

By Jennifer Nadel, MD
Medically reviewed checkmarkMedically reviewed
June 21, 2022

If you’ve ever worked out vigorously, you’ve probably experienced muscle fatigue.

As your exercise session progresses, actions you were once able to perform powerfully can suddenly become weaker and weaker—until your muscles cannot “fire” at all. 

There is no one cause of muscle fatigue, and there are still things we are learning about the causes and mechanisms behind it.

One thing that’s certain: Everyone experiences it when exercising, even the world’s best athletes.

For people who are looking to maximize their physical performance, certain treatments and supplements may help lessen its impact.

In this article, I’ll define muscle fatigue as it applies to everyday exercise.

I’ll outline the major causes and symptoms of muscle fatigue.

I’ll discuss the major ways to treat and delay muscle fatigue, including the use of at-home remedies and supplements.

And I’ll also discuss when to see a doctor about muscle fatigue.

What is Muscle Fatigue?

In medical literature, muscle fatigue is defined as a decrease in the power capacity of a muscle that occurs during exercise.

This definition of muscle fatigue encompasses two different scenarios that can result in lessened muscle power.

First, bodies fatigued by exercise may exhibit a lessened ability to signal, or order, muscles to fully contract. 

And second, for any given “contraction strength” that the body orders, a muscle’s actual contractions may become weaker and weaker as exercise fatigue sets in.

These two muscle fatigue scenarios can overlap during a single exercise session.

It all depends on the type of exercise being done, and the condition of your body.

Some people can exercise longer before experiencing muscle fatigue, while other people tire out more quickly.

Even a single person’s experience of muscle fatigue can vary day by day.

What isn’t included in the standard definition of muscle fatigue is muscle decline that’s caused by illness.

Some neurological, muscular and cardiovascular disorders, as well as inflammatory and autoimmune diseases, can cause severe and persistent muscle fatigue—and require immediate medical intervention.

Exercise-induced muscle fatigue, on the other hand, is generally relieved in a matter of days by rest and recovery alone.

Muscle fatigue is also different from muscle injuries, muscle soreness, and body aches.

While muscle fatigue is a decrease in power, muscle soreness is an increase in pain.

Experiencing muscle fatigue? Chat with a medical provider using K Health.

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Causes

There is no one cause of muscle fatigue.

Different exercise scenarios may trigger different pathways to muscle weakness.

Muscle fatigue may progress one way during aerobic exercise that draws heavily on respiratory muscles like the diaphragm, and another way during strength training sessions that put particular stress on skeletal muscles.

Doctors are learning more every day about the exact, cellular-level mechanisms behind muscle fatigue.

It’s already clear that there can, in fact, be multiple causes, including:

  • Not enough oxygen: For certain kinds of exercise, low levels of available oxygen, in the environment or within the body, can accelerate fatigue. Oxygen helps cells to break down a substance called glucose, which supplies muscles with the energy they need to “fire” and contract.
  • Not enough energy: When levels of glucose and other energy sources in muscle cells are too low, muscle fatigue can accelerate.
  • Impaired calcium release: The release of calcium molecules seems to play an important role in signaling skeletal muscle fibers to “fire” during exercise. As exercise progresses, the release of these calcium molecules seems to become impeded.
  • Metabolite and inorganic phosphate buildup: Metabolites are substances that are left over in cells as the body breaks down glucose and other energy sources. During exercise, the accumulation of these metabolites seems to play a role in increasing muscle tiredness. For instance, when it needs extra energy, the body often breaks down a substance called creatine phosphate—leaving behind a buildup of inorganic phosphate as a metabolite. This phosphate buildup seems to be a major cause of muscle fatigue, in part for the ways that it interferes with the calcium signaling system.
  • Not enough vitamins, minerals, or electrolytes: Significant deficiencies in substances like folate, iron, and vitamin B12, can make it harder for red blood cells to bring oxygen to muscles. Charged electrolytes such as calcium, magnesium, potassium, and sodium help the body signal muscles to contract.
  • Not enough water: Dehydration can slow and limit blood flow to muscles during exercise.

It was once thought that the buildup of lactic acid in muscles during exercise was an important cause of muscle fatigue.

It’s true that lactic acid accumulates as exercise progresses, and that in highly elevated qualities, lactic acid can cause lactic acidosis, which can cause burning sensations and muscle cramps.

There’s little evidence that lactic acid causes muscle weakness itself in the way that  inorganic phosphate buildup seems to directly cause fatigue. In fact, lactic acid may actually work to delay muscle fatigue!

Symptoms

The symptoms of muscle fatigue tend to follow a progressive course.

You are unlikely to go from total, full muscle power to complete muscle exhaustion in a short moment.

Instead, you’ll experience a gradual lessening of the power of your muscle’s contractions.

What this weakening looks and feels like depends on the kinds of muscles you’re exercising most heavily.

If you’re running fast, and thus giving your respiratory muscles a tough workout, you’ll likely experience muscle debility as the process of becoming more and more out of breath. 

If you’re exercising your arms or legs heavily through weight training, you may find it more and more difficult to complete your lifting sets as your muscle power decreases. 

If you’re doing advanced yoga, you might find your form slipping as stabilizer muscles grow more fatigued.

Treatment

Under normal circumstances, the best way to “treat” muscle fatigue is through rest and recovery.

This means not strenuously re-engaging tired muscles in the days after a heavy workout.

It also means building warm up and cool down sessions into all of your exercise sessions, and taking time to rest between workout sets. 

At-home remedies

In between heavy exercise sessions, you can also take many steps to both treat and prevent muscle fatigue.

Some of the most important remedies are sleep and a well-balanced diet.

Carbohydrates in particular are essential for ensuring your muscles have a sufficient supply of glycogen and other energy sources during exercise.

Hydration with water or sports drinks can also help to prevent dehydration and maintain electrolyte levels.

You can also work to build up your respiratory endurance and lung capacity through at-home aerobic workouts.

This ensures you will have an adequate oxygen supply the next time you put a lot of stress on your muscles.

Experiencing muscle fatigue? Chat with a medical provider through K Health.

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When to See a Doctor

While exercise-induced muscle fatigue should resolve with rest, more severe forms of muscle weariness may not.

They may instead be connected to a serious illness.

Contact your doctor immediately if previously easy physical activities now leave your muscles fatigued for long periods of time, or if you’re experiencing muscle heaviness alongside:

  • Debilitating body aches
  • Loss of mobility or difficulty walking
  • Difficulty swallowing, eating, or drinking
  • Trouble breathing or dizziness
  • Severe water retention
  • Persistent pain that does not improve with home treatments
  • Body aches related to a tick bite or unexplained rash
  • Fever (a temperature of 100° F/37.8° C or above)
  • Shortness of breath
  • Chest pain
  • A stiff neck
  • Sensitivity to light or other changes in vision
  • Extreme fatigue that does not get better with sleep
  • Fainting, seizure, or a loss of consciousness
  • Nausea and vomiting
  • Unexplained body aches that occur for more than two weeks
  • Weakness in your arms or legs

How K Health Can Help

Did you know you can get affordable primary care with the K Health app?
Download K to check your symptoms, explore conditions and treatments, and if needed text with a doctor in minutes. K Health’s AI-powered app is HIPAA compliant and based on 20 years of clinical data.

Frequently Asked Questions

What is an example of muscle fatigue?
One example of muscle fatigue is when you’re exercising your arms or legs heavily through weight training; as you progress, you will find it harder and harder to complete your sets, as your muscle contractions grow less powerful and fatigue sets in.
What does muscle fatigue feel like?
Muscle fatigue feels like the gradual weakening of your muscles’ power. This could be when you’re running and finding yourself growing more and more out of breath, or exercising your arms or legs and finding you’re able to drive less and less force through your limbs.

K Health articles are all written and reviewed by MDs, PhDs, NPs, or PharmDs and are for informational purposes only. This information does not constitute and should not be relied on for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.

Jennifer Nadel, MD

Dr. Jennifer Nadel is a board certified emergency medicine physician and received her medical degree from the George Washington University School of Medicine. She has worked in varied practice environments, including academic urban level-one trauma centers, community hospital emergency departments, skilled nursing facilities, telemedicine, EMS medical control, and flight medicine.