Lactic Acid: Friend or Foe?
It used to be a common belief that
lactic acid was a direct cause of fatigue and the major component responsible for muscular soreness or delayed onset muscle soreness (DOMS). Contrary to popular belief, lactic acid is not directly responsible for muscular fatigue or delayed onset muscle soreness. So, let’s set the record straight about what lactic acid is and what it actually does.
To understand what lactic acid does and how it works we muscle first define lactic acid and describe it in a general manner. In textbook terms lactic acid is the anaerobic end product of glycolysis, occurring when there is inadequate oxygen availability at the cellular level in the muscle (Power &Howley, 2009 and Williams, Anderson & Rawson, 2013). This means that when carbohydrates are broken down and utilized in the energy system – that is, predominantly used for high intensity short duration activities – lactic acid is produced.
As you will see in this article and much of physiology literature, the words lactate and lactic acid are used interchangeably. This is slightly confusing, but mainly the only difference is the dissociation of a H+ (hydrogen) ion form lactic acid, forming lactate, which will eventually be a contributing factor to fatigue (discussed in later paragraphs).
Now that you know a little bit more about what lactic acid is, let’s discuss what it can and dqoes do. What most people do not realize is that lactic acid is actually a form of energy that can be used during exercise. The human heart is the largest source of lactic acid utilization in the body. Yes, you heard that correctly. The heart actually is the body’s largest consumer of lactic acid.
The heart, being composed of a particularly unique type of slow-twitch muscle fibers, produces all of its energy aerobically, in the presence of oxygen. During exercise, or even during day-to-day activities, the muscles always produce some amount of lactic acid, which then enters into the bloodstream. The lactic acid then travels via the bloodstream and the liver. The heart then essentially performs the reverse process of the anaerobic glycolytic pathwa
y of which lactic acid originated from to begin with. It turns it back into pyruvate first and then back into ATP (the energy currency of cells). This newly “restored” energy can then be directly used by the cardiac muscle cells in the heart.
A similar thing actually happens in the liver as well. The liver uses a process called the Cori Cycle to convert lactate to glucose and then put it back into the bloodstream to be used as energy or used to make liver glycogen. Unlike muscle glycogen that is restored directly from blood glucose, the majority of the body’s liver glycogen is restored indirectly from lactic acid.
While we mentioned that lactic acid was not directly related to fatigue, it is, however, indirectly related to fatigue in one possible way. High levels of lactate in the muscles actually change the pH or acidity/basicity, due to the ionization and release of hydrogens that we mentioned previously.
By changing the pH in the muscle, it is believed the calcium handling, which is the major component in the contractile properties, is effected or disturbed. The increased number of hydrogen ions in the muscle can also inhibit enzymes that play a key role in the aerobic and anaerobic production of ATP (Powers &Howley, 2009).
There are also many oth
er theories for fatigue. This is due to the wide variety of factors and variables that physiologically can contribute to actual fatigue, thus, making it extremely difficult to pin point a single cause. In regards to the comment of lactic acid causing the delayed onset muscle soreness after an exercise bout, it is actually now believed that the cause of DOMS is “reversible structural damage” (Williams, Anderson & Rawson, 2013) done at the cellular level in the muscle.
An interesting thing to note about lactic acid is also what happens when athletes train at altitude. Say you are a bodybuilder traveling to Denver, which has a much higher elevation than sea level where you live. With acute training (no acclimation to the altitude) in hypoxic conditions (low oxygen levels) there is an elevated lactic acid response and with chronic training (after 3-4 weeks at acclimation to altitude) the lactic acid response to the same amount of work or load is far less than the acute training response. This phenomena – called the “lactate paradox” – is not entirely understood yet but is thought to be “the result of both hormonal (epinephrine) and intracellular (lower [ADP]) adaptations that occur with chronic exposure to hypoxia” (Powers &Howley, 2009).
While this was a rather simple and more watered down approach to the physiology of lactic acid production and use, I hope that it provided you with a little more insight on the topic in question. It is always important to know what goes on at the cellular level in order to determine the systemic effects. This is especially important in relation to exercise.
Sources & Photo Credits
Powers, S. K., &Howley, E. T. (2009).Exercise physiology theory and application tofitness and performance. (Seventh Edition ed.). Boston: McGraw-Hill.
Sherwood , L. (2006). Fundamentals of physiology a human perspective. (Third Editioned.). Belmont: Thomson Brooks/Cole.
Williams, M. H., Anderson, D. E., & Rawson, E. S. (2013).Nutrition for health, fitness &sport. (Tenth Edition ed.). New York: McGraw-Hill.
http://www.delano.k12.mn.us/high-school/academic-departments/science/mr-b-wiesner/cross-country/10-things-you-should-know-about-lactic-acid
http://jiujitsumania.com/the-connection-between-combat-sports-and-aerobic-anaerobic-training/
http://www.bodyrecomposition.com/training/why-the-us-sucks-at-olympic-lifting-oling-part-2.html
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