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.

Fuel

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 start 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.

Calcium

One of the functions of calcium is to help control muscle contractions. Researchers at the Columbia University Medical Centre, Bellinger AM 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 rest.

Referenced Material

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

Page Reference

The reference for this page is:

• MACKENZIE, B. (2007) Muscle Fatigue [WWW] Available from: http://www.brianmac.co.uk/musclefatigue.htm [Accessed

Associated Pages

The following Sports Coach pages should be read in conjunction with this page:

• Blood Pressure
• Energy Pathways
• Maximum Heart Rate
• Physiology - Articles
• Physiology - Literature Reviews
• Physiology - Cardiovascular System
• VO2 max
• vVO2max and tlimvVO2max