Fit Fridays with Holly Lowe Jones: Understanding exercise-induced fatigue


Can you run 10 miles before fatigue starts to set in, or do you feel tired after only a few minutes of exercise? Understanding exercise-related fatigue can go a long way in helping you get in your personal best shape.

After recently moving across the country, I was unable to compete in my favorite sport with the same regularity that I have in prior seasons. As I approach middle age, I am increasingly aware of the effects of aging on my level of fitness. Training for the 2015 triathlon season has officially begun, and I am constantly taking inventory of how tired I feel before, during and after training. I decided to do some research to find out how to assess my energy level as it relates to exercise, and here’s what I learned:

Fatigue, or getting tired from exercise is best looked at on a continuum. If you are doing an all-out sprint on a bicycle, you will reach fatigue that is different than getting tired from a long, slow bike ride. Fatigue is a continual decrease in muscular performance while exerting the same effort, marked by feelings of tiredness. Fatigue ultimately causes muscle failure at a given intensity, despite the same power output or determination.

Fatigue is not to be confused with muscle damage and weakness. Fatigue is best solved by rest. During my triathlon season, my body is in a constant state of repair; muscle soreness feels normal, and I can easily sleep for 10-12 hours a day. For serious competitive athletes, it is important to minimize fatigue. In Physiology of Sport and Exercise the authors write, “Ask most exercisers what causes fatigue during exercise, and the most common two-word answer is ‘lactic acid.’ Not only is this common misconception an oversimplification, but there is mounting evidence that lactic acid may actually have beneficial effects on exercise performance!”

Despite this frequent, yet false, explanation for muscle fatigue, lactic acid is actually used as fuel during exercise and not the primary cause of fatigue. In a process called the Cori cycle, lactate is recycled into glucose for use as an energy source. This is a necessary process in order for prolonged exercise to occur.

So, what really causes fatigue or “hitting the wall,” as it is commonly referred to among endurance athletes, particularly distance runners?

In 2005, I ran my first marathon. My first 18 miles, I was cruising at an 8:30 pace, until somewhere between mile 21 and 22 I slowed dramatically. At the time, I didn’t know that this is common in marathon running. My quad muscles ached, and my legs felt heavy and sore. I simply couldn’t turn them over at the same rate any longer. This type of muscle fatigue is also known as peripheral fatigue and is caused by a combination of physiological changes. There is a decreased rate of energy delivery, and an accumulation of metabolic by-products (such as lactate) leads to diminished neural control. The result is muscle contraction failure. In addition, central fatigue occurs when changes in the brain cause motor unit level alterations. All these components work together to cause the experience of fatigue.

Fatigue also depends on what type of exercise is being performed. Different activities require the use of different muscle fibers. Individual level of fitness and diet will also affect the rate and intensity of muscle burn-out. Understanding what causes fatigue can help us to avoid and prolong it with proper preparation, hydration, nutrition and rest.

Wishing you health and happiness,

Holly Lowe Jones is a media professional, fitness expert, and ISSA-certified personal trainer. A member of the National Association for Health and Fitness, Jones is also a seasoned triathlete who competes in her spare time.

For more information, please visit her website


Instagram and Twitter: HollyLoweJones

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