What Causes Fatigue In High-Intensity Endurance Sports?

For endurance athletes, fatigue is perhaps the single most important performance factor to understand. A new study explores the role of metabolic factors and temperature in fatigue onset.

General Patton once said, “Fatigue makes cowards of us all.” It also kills performance. For endurance athletes, fatigue is perhaps the single most important performance factor to understand. Performing at your peak with the least possible fatigue means you cross the finish line faster. Fatigue yourself too much in a training session, and you won’t be able to train as hard the next day, if at all. According to a recent study in the Journal of Strength and Conditioning Research, fatigue for some kinds of endurance sports isn’t as well understood as we may think.

Before going further, let’s define fatigue, since it’s an ambiguous term. Fatigue in the technical sense means something a bit different than how we might mean it in conversation. When we say fatigue, we often mean perceived exertion. We describe fatigue based on how it feels, because that’s usually the best measurement tool we have in training besides performance itself. To be precise, however, fatigue is defined by science as the point when you can no longer maintain a given output of force. For a lot of sports, maintaining force output is the name of the game. When fatigue sets in, you have to either slow down or crank out all your effort just to maintain your pace.

In this study, the researchers compared the effects of four conditions on cyclists:

  • 80% of VO2 max at 100 degrees Fahrenheit (37 degrees Celsius)
  • 100% of VO2 max at 100 degrees Fahrenheit
  • 80% of VO2 max at 50 degrees Fahrenheit (10 degrees Celsius)
  • 100% of VO2 max at 50 degrees Fahrenheit

These are fast paces, especially on a bike. In each condition, the participants rode to exhaustion while the researchers took their blood and their temperatures for analysis.

The researchers chose these four conditions due to the nature of fatigue. There are many proposed causes of fatigue, from neurological to mechanical. But the truth is, fatigue is a product of many factors. The researchers hoped to get an idea of what the fatigue-causing mechanisms might be in each of these conditions, particularly because most research to date looked at slightly less intense cardio.

Unsurprisingly, the intensity of the exercise seemed to be a more significant factor than the temperatures used in the study. Both of the 100% VO2 max groups tired faster than the 80% groups. The 100% groups all performed at about the same ability, regardless of the temperature. In the 80% groups, however, temperature was a significant factor. The 80% group that rode in the cool room performed better than the 80% group in the hot room. In fact, despite the same VO2 max in the lower intensity groups, the time to exhaustion was doubled in the cooler room. That’s a huge difference.

The higher intensity groups had more acidic blood at exhaustion, whereas the lower intensity groups had higher body temperatures at exhaustion. These two facts alone point to the different possible causes of fatigue. The researchers hypothesized that metabolic factors played a major role in fatigue at 100% VO2 max, while body temperature played the biggest role at 80% VO2 max.

The researchers didn’t examine neural sources of fatigue in this study, and they noted that their findings indicate neural factors probably played an additional unquantifiable role in the results. Although more study is needed, this study sheds some light on possible fatigue-reducing strategies at each of these intensities.


1. Joel B. Mitchell, et. al., “Fatigue during high intensity endurance exercise: the interaction between metabolic factors and thermal stress, Journal of Strength and Conditioning Research, DOI: 10.1519/JSC.0000000000000319

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