What causes muscle fatigue during workouts

 September 22, 2021|  Dona Maria

If you want to transform your physique, it’s crucial to push yourself during every workout. High-intensity training can help you get bigger, stronger, faster—whatever the goal. But, when we exert ourselves with the type of intense training required to produce head-turning results, our bodies have mechanisms that trigger fatigue. Not only is it an evolutionary response to protect us from the stress of intense physical exertion, but it’s also a sign that the fuel tank is empty. While it’s natural for energy levels to drop as a workout progresses, there are key factors contributing to fatigue during a workout—and some things you can do to fight it.

Hydrogen Buildup

During a workout, the body uses different substrates as fuel. For high-intensity exercise such as a CrossFit WOD or dropsets during a bodybuilding workout, near-maximal efforts are required, and acidosis within muscle tissue is often a limiting factor. You know that feeling when you’re knee-deep in a high-rep set and it becomes difficult to crank out those last few reps because your muscles feel like they’re on fire? That burning sensation is the buildup of hydrogen and other byproducts in your muscle.


Glucose is the go-to fuel source for high-intensity training because the body can quickly break it down and convert it to adenosine triphosphate (ATP). The breakdown of glucose, a process known as glycolysis, generates compounds such as carbon dioxide and hydrogen as byproducts. As hydrogen ions accumulate, the pH levels in the body are altered, which results in a more acidic environment. The higher body pH affects overall performance by altering the muscles’ ability to contract, causing fatigue. For athletes who push their training to the limit, excessive hydrogen buildup can be one of the reasons their workouts are cut short.

Fortunately, the body has built-in systems that work to maintain balance and neutralize the acid buildup. One of these molecules is carnosine, a dipeptide made up of the amino acids histidine and beta-alanine. Carnosine helps increase the muscle’s pH by buffering hydrogen ions, allowing for better overall muscle function.

The Fatigue Factor

Central fatigue refers to the neural-fatiguing effects of exercise that are mediated through the central nervous system (CNS). Fatigue builds up during exercise and begins to normalize post-workout. Some researchers suspect that central fatigue is a protective mechanism that the CNS uses to prevent injury, by decreasing neural drive to the muscle.

As central fatigue builds, the firing rate of muscles declines, and the rate of perceived exertion increases. Lifters can feel this effect when they’re at the end of a hard set, when each rep feels progressively more difficult, despite the barbell weighing the same throughout the set.

Ammonia Production

Skeletal muscle releases ammonia during exercise, and exercise scientists have long suspected that it is a contributing factor to fatigue. Researchers originally believed that ammonia buildup only occurred at higher intensities, upward of 80 percent of VO2 max. That’s because ammonia production increases as the high-intensity work rate increases. In some individuals, however, ammonia production was observed with intensities as low as 60 percent of VO2 max.

Exercise causes an increase in adenosine monophosphate, which gets deaminated and converted into ammonia. Increased ammonia concentration has been associated with increases in fatigue because of its role in accelerating glycolysis and its impact on the CNS.

One mechanism through which ammonia is cleared from the body is the urea cycle. L-citrulline is an amino acid involved in the urea cycle that can buffer ammonia, and some researchers speculate that is one of the factors contributing to citrulline’s performance-enhancing effects.

If you’ve been trying to push the intensity and output of your training but are finding yourself struggling to maintain your strength and energy, it could well be linked to one or more of these fatigue-inducing reactions.

 September 22, 2021 | Dona Maria
Dona Maria

About the Author

Dona Maria Mesmar is the Nutrition Manager for Kcal Brands. She develops nutrition and diet programs for athletes and provides them with one on one nutrition consultation to help them achieve their body composition goals. She is also a certified fitness instructor, published nutrition writer and has appeared numerous times on national live TV, where she has provided expert advice on nutrition and healthy eating.

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