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Muscle Fatigue

We use the term fatigue to describe a general feeling of the overall effects of an exercise on the body or the inability to continue with an exercise. So what causes this sensation of fatigue? The reason has to do with several factors including the availability of fuel for the muscles and the mechanism of hydrogen ions and calcium in muscle cell action.


Muscles require ATP (adenosine triphosphate) as an energy source. During intense activity, we rely on the anaerobic pathway, but this has a limited store (ATP/CP pathway approximately 10 seconds and the Anaerobic Lactic pathway approximately 2 minutes). The aerobic pathway produces ATP copiously (with the breakdown of glucose and glycogen) and requires oxygen, carried by the blood, to support the process. The cardiovascular system is limited in its ability to deliver blood and oxygen to the working muscles.

Hydrogen ions

The breakdown of glucose or glycogen produces lactate and hydrogen ions (H+). If insufficient oxygen is available to the working muscles, then hydrogen ion concentrations increase, and the blood and muscle become acidic. This acidic environment starts to block the nerve signals from the brain to muscle fibres, so the legs begin to feel heavy and we slow down in order to allow more oxygen to get to the working muscles.


One of the functions of calcium is to help control muscle contractions. Researchers at the Columbia University Medical Centre, Bellinger et al. (2008)[1], conducted a study which found that muscle fatigue after long intense exercise may be caused by tiny leaks of calcium inside muscle cells. The researchers found that after extended high-intensity exercise, 3 hours of cycling by experienced cyclists, small channels in the athlete's muscle cells were leaking calcium. This calcium leak weakens muscle contraction and stimulates an enzyme that attacks muscle fibres resulting in muscle fatigue. These calcium leaks stopped after a few days' rests.


Fibromyalgia is pain and stiffness in the muscles, ligaments and tendons which may affect one part of the body or several different areas such as the limbs, neck and back. You will have a greater risk of developing fibromyalgia if one of your parents or siblings has the condition. It is thought fibromyalgia is caused by a combination of genetic and environmental factors work together.

Some of the symptoms are fatigue and exhaustion, sleep disturbance, aching and stiffness, headaches, concentration problems and irritable bowels. Any of these symptoms can make it hard to carry out simple everyday tasks which can then make you feel frustrated and depressed.

There appears to be no simple cure for fibromyalgia, but there are ways of managing your symptoms. Aerobic exercises, such as swimming and walking, will reduce pain and fatigue which could help you to sleep and feel better.

For more information visit the UK Fibromyalgia Association


  1. BELLINGER, A.M. et al. (2008) Remodelling of ryanodine receptor complex causes "leaky" channels: A molecular mechanism for decreased exercise capacity, The Proceedings of the National Academy of Sciences, 2008

Related References

The following references provide additional information on this topic:

  • FITTS, R. H. (1994) Cellular mechanisms of muscle fatigue. Physiological reviews74 (1), p. 49-94
  • EDWARDS, R. H. (1981) Human muscle function and fatigue. Human muscle fatigue: physiological mechanisms, p. 1-18
  • CHAFFIN, D. B. (1973) Localized muscle fatigue-definition and measurement. Journal of Occupational and Environmental Medicine15 (4), p. 346-354

Page Reference

If you quote information from this page in your work, then the reference for this page is:

  • MACKENZIE, B. (2007) Muscle Fatigue [WWW] Available from: [Accessed

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