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What Are the Causes of Muscle Fatigue?

By Anthony Diaz de la Vega

Muscle fatigue is a normal byproduct of exercise but can also indicate an injury. Knowing how to tell if you are experiencing simple muscle fatigue or an injury can mean the difference between muscle growth and an ankle brace.

Muscle Fatigue

Muscle fatigue is the phenomenon of pain or weakness in a muscle after a round of exercise. While exercising, your muscles develop microscopic tears that must be repaired in order for the muscle to grow in strength and size. One good thing about muscle fatigue is that it is fairly predictable and recognizable; its duration also provides a good basic indicator of when it is safe for you to work the sore muscle again.

Common Causes

The most common cause of muscle fatigue is exercise; it usually indicates that your workout was able to challenge your muscles adequately to require rest. For fatigue following exercise, proper nutrition and rest are recommended, and you can take an over-the-counter painkiller if your muscles are especially sore.

Another cause of fatigued or weakened muscles is an injury. Mild injuries do not feature sharp, recognizable pains. Often an injury shows as dull pain in a generalized area, similar to exercise-induced muscle fatigue. If you exercised a muscle group longer than three days ago and are still experiencing discomfort and lack of strength, have a health care professional check you out for an injury.

Chronic Muscle Fatigue

In addition to exercise and injury, there are a host of chronic medical conditions that affect muscles or motor control, leading to muscle fatigue or weakness. The most common medical conditions that can cause muscle fatigue are colds and influenza; contracting a bug forces your body to put most of its resources into fighting the bacteria or virus, leaving little energy for muscular strength or endurance. Thyroid disease (whether it is an overactive or under-active thyroid) is also a very common cause of muscle fatigue and general lack of strength or energy.

Lifetime chronic illnesses like fibromyalgia, Lou Gehrig's disease and cerebral palsy all limit muscular function, either by affecting the muscle tissue or the motor cortex. These forms of muscle fatigue tend to be elusive and hard to treat, and are often diagnosed early on in life.


Muscle fatigue is not necessarily a bad thing. If you monitor it carefully, fatigue helps you identify your personal limits of exercise and make the most of your rest time. On the other hand, injuries can oftentimes be disguised as simple fatigue, leading to overwork of muscle and perhaps a more severe injury.

Benefits of Fatigue

Muscle fatigue is indicative of a good workout. Challenging your muscles is the only way to add muscle mass and improve your physique, so fatigue should be a welcome sign in your workouts and recovery. Also, muscle fatigue provides fairly reliable "pain map" of your body that lets you know which muscles are still recovering and which are safe to work again.

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