The misconception that lactic acid creates muscle fatigue has circled society for many years. Some people believe that when their muscles fatigue during exercise, it is primarily due to a build-up of lactic acid. “I can feel the lactic acid burning in my legs” is a common phrase that people might say when exercising. But is this really the case? Does lactic acid really cause muscle fatigue or is it just a myth? Research demonstrates how lactic acid’s harmful effect towards muscle fatigue is another fallacy.
What is lactic acid? Lactic acid is created and utilised in muscles when there is a significant energy demand, rapid changes in energy requirement, and/or inadequate amounts of oxygen supplied to the working muscles (Sahlin, 1986). Simply put, lactic acid is made and used in the exercising muscles primarily during anaerobic activity.
What is the anaerobic energy system? The anaerobic energy system refers to the process whereby ATP is created without oxygen (Gastin, 2001). More specifically, the lactic/glycolytic system (part of anaerobic respiration) is the predominant energy system used when the duration of exercise lasts between 10-60 seconds and intensity is relatively high.
Although this is the case, it interplays with exercise that lasts less than 10 seconds and more than 60 seconds. An example of this is a marathon that lasts greater than 2 hours. While the aerobic energy system is predominant, there is still a slight contribution (about 2%) from the anaerobic energy system (lactic/glycolytic energy system more specifically) during a marathon.
Is lactic acid the cause of muscle fatigue? Many people that are not well educated in physiology believe that lactic acid is the main reason why our muscles fatigue during exercise and after exercise. Lactic acid is actually not the cause of muscle fatigue. Westerblad, Allen and Lannergren (2002) suggest that recent studies display an insignificant link between lactic acid accumulation and muscle fatigue. In other words, a build-up of lactic acid doesn’t mean that it is the cause of muscle fatigue. It would be similar to say that the larger your foot size the faster you will be at sprinting. There is no direct link between the two variables.
In reference to muscle fatigue and/or muscle soreness after exercise, some people may believe that lactic acid is the cause of Delayed-Onset Muscle Soreness (DOMS). However, Schwane, Watrous, Johnson, and Armstrong (1983), claim that lactic acid has no relation to exercise-induced DOMS. Even though subjects in their study had significantly increased lactic acid levels during treadmill running, there was no significant post-exercise muscle soreness. In addition to this, the subjects who completed downhill running had no elevated lactic acid levels, but experienced major exercise-induced DOMS. This displays once again how a change in lactic acid during exercise doesn’t necessarily mean that it will affect muscle fatigue or soreness.
In fact, lactic acid can have a positive effect on our muscles. According to Cairns (2006), some tests on isolated muscle suggest that lactic acid can decrease muscle fatigue during high intensity exercise. “Lactic acid influences the activity of chloride ion channels, which in turn sustains the action potentials that are necessary for muscle contractions” (Allen & Westerblad, 2004, p. 1112). Put simply, lactic acid assists in allowing action potentials to occur which is an important process for our muscles to contract.
What is the cause of muscle fatigue? Some individuals may argue that lactic acid still contributes to muscle fatigue even at a small percentage. Cairns (2006) claims that lactic acid has a slight negative effect on muscle fatigue. However, “lactic acid is not a crucial factor in the development of fatigue” (Lamb & Stephenson, 2006, p. 1413). It is fair to say that lactic acid increases during high intensity exercise; although it doesn’tnecessarily mean that it is a major cause for muscle fatigue.
Research has discovered other factors that are the primary contributors towards muscle fatigue. Sahlin (1986) suggests that an increase in inorganic phosphate is a key factor in the production of muscle fatigue. In addition to this, Lamb and Stephenson (2006) also suggest that muscle fatigue occurs due to a disturbance in excitation-contraction coupling (EC Coupling). Some of these disturbances include an increase in metabolites (inorganic phosphate, adenosine diphosphate/ADP, magnesium) and a decrease in substrates (adenosine triphosphate/ATP, creatine phosphate/Pcr, and glycogen) in the muscle fibres.
Conclusion The belief that lactic acid is the cause of muscle fatigue is incorrect. Lactic acid is made and used in the exercising muscles primarily during anaerobic respiration. Although lactic acid increases during exercise, it doesn’t increase muscle fatigue as there is an insignificant relationship between the two variables. The main causes of muscle fatigue are due to an increase in metabolites (inorganic phosphate, ADP, and magnesium) and a decrease in substrates (ATP, Pcr, and glycogen) in muscle fibres.
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Westerblad, H., Allen, D. G., Lannergren, J. (2002). Muscle Fatigue: Lactic Acid or Inorganic Phosphate the Major Cause? In: News in Physiological Sciences. 17(1), 17-21
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