While exercising, your muscles feel weak, painful and tired. This is muscle fatigue, and it can be caused by a variety of physical, environmental, biochemical and nutritional factors. Writing in the December 1998 issue of the "European Journal of Applied Physiology", Jane Kent-Braun notes that muscle fatigue can be caused either by a failure of the nervous system to communicate with muscle tissue or by metabolic processes.
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Muscles need energy to contract and move a part of your body, and cells need oxygen to create this energy for muscle contraction. When the oxygen supply is plentiful, the cell respiration process is known as aerobic respiration. When oxygen is not available, cells respire anaerobically, and lactic acid is produced. Lactic acid is a waste product which causes muscle pain and fatigue, according to The Stretching Institute. If you have been exercising and are out of breath or hyperventilating, this will lead to anaerobic respiration and therefore lactic acid-induced muscle fatigue.
Certain minerals and electrolytes are necessary for proper muscle function. The Stretching Institutes suggests that, if you often experience unexpected muscle fatigue and cramps, you may be deficient in certain key minerals such as potassium, magnesium and calcium. These minerals are vital for muscle function -- if you feel that your muscle fatigue is caused by deficiency in one or more of these minerals, look for a way to increase dietary intake of these minerals.
Failure to Stretch or Warm Up
If you go straight into strenuous exercise without a thorough warm-up, your muscles are likely to be surprised by the sudden activity and respond with cramps and fatigue. Stretching is important before and after any rigorous exercise in order to avoid muscle fatigue. This will keep your muscles loose and increase your overall flexibility over time.