In the context of exercise, muscle fatigue is when your muscles are so tired that you can't continue at your current intensity. It happens because of a complex chemical process inside your cells that depletes your energy. How fast you experience muscle fatigue depends on your fitness level, exercise intensity, and environment. Genetics may also play a role. An AncestryDNA^® + Traits test can tell you whether people with DNA like yours have muscles that tire at an above-average, average, or below-average rate.

The Science Behind Muscle Fatigue

During a short, all-out-effort, like weightlifting, your muscles get energy from adenosine triphosphate (ATP), which is inside your cells. But ATP runs out quickly—in as little as 10 seconds—and that leaves your cells scrambling to get fuel. As a result, substances like lactate and hydrogen ions build up in your cells. This causes that burning, painful feeling that makes it nearly impossible for muscle contractions to occur as needed.

During lower intensity types of exercise, like cross-country skiing, your muscle function uses glycogen as fuel. But it eventually runs out, too—usually within 2 hours. Unlike with ATP, you can replenish glycogen by taking in carbohydrate-rich food or liquids. That's why someone can run for 24 hours, but not sprint for 24 hours.

How to Improve Muscle Fatigue

Muscle fatigue happens to everyone, from those new to exercise to seasoned athletes. If your muscles didn't experience fatigue, you wouldn't know when to pull back and rest.

There are a few ways you can influence how efficiently your muscles function. For example, there's a reason runners eat pasta the night before a race: fueling up on carbohydrates can help you maintain your glycogen stores. Drinking plenty of water can also help prevent muscle fatigue. The 8-glasses-per-day rule is a good one, but you likely need more when exercising, especially in the heat. Building up your endurance by doing interval training—repeated bouts of intense exercise with rest in between—can also help to improve recovery from muscle fatigue.

Muscle Fatigue and Genetics

One gene that researchers have found that strongly affects muscle fatigue is the MCT1 gene. This gene helps transport lactate, which is one of the factors in how quickly your muscles give out and stop being able to contract. Being able to clear lactate from your bloodstream slows muscle fatigue.

Which allele, or version, of this gene you have, can influence how your body deals with muscle fatigue. Compared to control groups, sprint and power athletes were more likely to have the TT genotype of MCT1. However, genetics are only one factor contributing to muscle fatigue. Also, discussions of genetic markers for muscle fatigue related to exercise shouldn't be confused with genetic disorders that cause general muscle weakness, like muscular dystrophy.

Interesting Facts About Muscle Fatigue

Kids and athletes may have a lot in common when it comes to the rate of muscle fatigue. An Australian study had kids, endurance athletes, and non-athlete adults do a series of intense physical activities. The study found that the kids' muscles fatigued at about the same rate as the endurance athletes. The theory is that kids don't produce as much lactate as adults. And their bodies also clear lactate more quickly, meaning that their muscles recover faster.

Athletes who are looking for every (legal) advantage may use various herbs and supplements thought to reduce muscle fatigue, including garlic, fish oil, creatine, and caffeine. Some athletes even swear by so-called energy drinks because of their mixture of carbohydrates and caffeine.

References

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