Lactic Acid: Friend or Foe?

For decades the main culprit of fatigue and the thorn in the side of many athletes was a foe called lactic acid. It has been called a waste product of anaerobic metabolism and has been believed to be responsible for the uncomfortable “burn” of intense exercise and directly responsible for the metabolic acidosis of exercise, leading to decreased muscle contractility and ultimately cessation of exercise. Unfortunately, for decades we have been misguided. We now know, lactic acid, or more correctly termed lactate, is correlated with fatigue but not a causative factor. This doesn’t mean that our training plans were all wrong or that we need to drastically alter how we improve our player’s fitness levels. It just means we have been misguided in what we were trying to improve and the implications of our training methods.

Introduction

For decades the main culprit of fatigue and the thorn in the side of many athletes was a foe called lactic acid. It has been called a waste product of anaerobic metabolism and has been believed to be responsible for the uncomfortable “burn” of intense exercise and directly responsible for the metabolic acidosis of exercise, leading to decreased muscle contractility and ultimately cessation of exercise. Unfortunately, for decades we have been misguided. We now know, lactic acid, or more correctly termed lactate, is correlated with fatigue but not a causative factor. This doesn’t mean that our training plans were all wrong or that we need to drastically alter how we improve our player’s fitness levels. It just means we have been misguided in what we were trying to improve and the implications of our training methods.

As we now know lactate is correlated with fatigue, so that as fatigue increases so does lactate levels increase. So if lactate is not the culprit, then who is? As fatigue increases so too does the build-up of accompanying byproducts such Hydrogen ions (H+) resulting in a direct linear relationship between it and lactate levels. One of the keys to performance in many sports is to delay the build up these accompanying products. If the rate of those products accumulation can be decreased, fatigue can be delayed. So while lactate is not the culprit, it corresponds with the build up of by-products that can cause fatigue. Strength & Conditioning coaches commonly use sessions to enhance lactate tolerance or the lactate threshold. While the traditional method of intensive intervals for lactate tolerance and extensive intervals using tempo runs for lactate threshold is very effective, knowing the underlying mechanisms of these components will help coaches design better sessions to enhance those variables.

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Lactic Acid Versus Lactate: An Important Differentiation

Despite the ubiquitous use of the term “lactic acid” in both scientific and lay fitness and sports medicine communities, the actual presence of meaningful quantities of lactic acid in the human body has been called into question. It is true that the glycolytic production of lactate is associated with hydrogen ion (Hþ) production, as represented in the following summary equations (Robergs et al., 2004):

Glucose → 2 lactate + 2 H+

Glycogen → 2 lactate + 1 H+

However, as detailed in the 2004 review of the biochemistry of exercise-induced metabolic acidosis by Robergs, Ghiasvand, and Parker, these summary equations do not imply that lactate is the source of H+, but rather that the proton release of glycolysis is likely associated with the non-mitochondrial hydrolysis of adenosine triphosphate (ATP). Although other explanations for H+ formation have been proposed, most investigators now agree that lactic acid is not produced in muscle (Lindinger et al, 2005). Although the construct of “lactic acidosis” appears intuitive and continues to be propagated in physiology texts and Strength & Conditioning education, no convincing evidence exists in support of this theory. Regardless of whether this stance represents as “sloppy nomenclature” as suggested by Lindinger et al (2005) or a true inherent misunderstanding of lactate’s production, it undoubtedly leads to confusion among many Strength & Conditioning coaches. For this reason, we will only use the term lactate.

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By-Product Build Up

In games based sports like Soccer and Rugby or Track & Field events like 400m and 800m, there is a significant anaerobic demand. Due to this anaerobic demand, certain by-products will accumulate in the body. For example, if the sprints are short but repeated in a game of Soccer, creatine levels will build up due to the demand on the ATP-CP for energy supply (ATP production). However, if the sprints become longer with the ball in play for longer periods and less time to recover, H+ levels will increase due to the greater demand on the Anaerobic Glycolysis system for energy supply.

Previous studies have demonstrated that an increase in H+, which is a proton that dissociates from lactate and would decrease the pH, may impair muscle contractility (Mainwood & Renaud, 1985). While the previously accepted notion that lactate played a direct role in fatigue, essentially acting as a “poison” to the muscles, has been disproved, it does not discount the entire acidosis concept of fatigue (Noakes, 2007). While lactate itself may not cause fatigue, it corresponds with other products in the body which may have contribute to fatigue, thus lactate can still be used in studies as a marker of fatigue. For instance, an increase in circulating blood lactate corresponds well with a decrease in pH and increase in H+. An increase in H+ has been shown to reduce the shortening speed of a muscle fiber, while a reduction in pH impairs Ca2++ re-uptake in the Sarcoplasmic Reticulum and may inhibit phosphofructokinase (PFK) (Hargreaves & Spriett, 2006; Brooks et al. 2004). In addition, a decrease in pH could stimulate pain receptors (Brooks et al., 2004). All of these actions could potentially cause fatigue.

The ability of the muscle to buffer the H+ could potentially delay fatigue. As mentioned above, a rise in H+ concentration causes many processes that could potentially lead to fatigue. Training has been shown to increase buffering capacity in both recreational and well trained athletes (Laursen & Jenkins, 2002). Furthermore, in one study, the buffering capacity of 6 elite cyclists was found to be significantly related to their performance in a 40km time trial (Weston et al., 1996). This demonstrates the importance of dealing with by-products and buffering capacity when it comes to performance. Due to the impact of acidosis on energy production and performance, much of the coaching and scientific literature focuses on delaying this process. Of particular interest is the concept of the lactate threshold, which will be discussed in our next follow up article.

References:

  1. Robergs RA, Ghiasvand F, Parker D. Biochemistry of exercise- induced metabolic acidosis. Am J Physiol Regul Integr Comp Physiol 2004;287:R502-R516.
  2. Lindinger MI, Kowalchuk JM, Heigenhauser GJ. Applying physico- chemical principles to skeletal muscle acid-base status. Am J Physiol Regul Integr Comp Physiol 2005;289:R891-R894; author reply R904-R910.

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