When we attempt to begin to understand lactic acid, most think of the burning in our legs during a 20 RM squat, or maybe Arnold’s famous line about the ‘Pump’. While these examples wouldn't be entirely incorrect, they are just not completely accurate. Exercise scientists long ago identified lactic acid and how it is produced. Where things get interesting is as it relates to performance and effects on the body. Many of the top minds in this ever evolving field of S&C are still arguing about how it is used by the body and its effects on muscle, strength and the body's ability to generate force.
Does lactic acid lead to fatigue?
Is it responsible for muscle growth?
Is it one of the human body’s many self defense mechanisms?
Many of the ideas surrounding lactic acid once thought to be true have been proven wrong by recent research and athletic feats. This article will take a look at how lactic acid is produced, several of the top theories of what the ‘pump’ is all about, as well identify some considerations for training athletes.
How is Lactic Acid Produced?
We are all familiar with the three metabolic pathways; Phosphagen, glycolytic, and oxidative. All three of these pathways fuel muscle contraction with adenosine triphospate (ATP) breakdown during exercise at any intensity. Each metabolic stage regenerates ATP differently, with different byproducts for each. Lactic acid has long been thought to be one of the byproducts of the glycolytic pathway and cause of the burning sensation or cramping during training and competition. Diving deeper into the physiology lesson, let's breakdown the glycolytic pathway to find out where exactly lactic acid is produced.
The glycolytic pathway has two processes for breaking down and regenerating ATP stores, anaerobic glycolysis and aerobic glycolysis. The first step for glycolysis is breaking the glycogen stored in muscles down to glucose which is then metabolized to pyruvate and NADH. These byproducts then take one of two paths. When there is sufficient oxygen available they travel to the mitochondria of muscle cells to enter the Kreb cycle for aerobic glycolysis (4). When there is an oxygen deficit or need for immediate energy, the pyruvate and NADH will be produced faster than they can be transported into the mitochondria. These two agents will then combine to form lactic acid in the cytoplasm of muscle fibers. Pyruvate takes up a hydrogen ion (H+) from NADH to become lactic acid, and NADH is oxidized to NAD+. How quickly the accumulation of lactic acid and NAD+ depends on the ATP demand of a given task and the ability of the athlete’s phosphagen system and aerobic glycolysis metabolism to meet this demand (3).
Pyruvate + NADH = Lactic Acid (pyruvate + H+) + NAD+
Theory of Acidosis
Now that we’ve introduced how lactic acid is produced and the agents involved, let’s cover some theories as to the effects. First we will cover the theory of acidosis developed by the first ever exercise physiologist, A.V. Hill, in the first half of the 20th century. Hill proposed that lactic acid accumulates in working muscles during exercise, increasing pH of the muscle fibers, normally 7.04, and reducing it to an acidic value ranging between 6.9 and 6.4. This condition is called acidosis. Most of the general knowledge of lactic acid across most high school football coaches is derived from Hill's findings and theory of acidosis.
Exercise physiologists have worked to support this theory since it was developed by Hill in the 1920’s. Research has been presented with evidence showing acidosis slows the rate of anaerobic glycolysis ATP production reducing muscle force and power by interfering with actin and myosin function. Several studies throughout the 20th century show a reduction of power in both muscle groups and single muscle fibers. The biopsies supported the theory of acidosis showing increased amounts of lactic acid in the muscles and blood of athletes when they became fatigued.
But does this mean lactic acid causes the burn and fatigue?
Not quite. Recent research has shown that in order for lactic acid to be removed from the muscle it must be converted to lactate. The conversion causes a hydrogen ion (H+) to be released from lactic acid it gained in conversion from pyruvate, and gains sodium (Na+) or potassium (K+) to leave muscle cell. During exercise, the build up of H+ electrical charges from the lactate conversion increase the acidity of the blood in the muscle cells (3). Lactic acid is not to blame for the burn and fatigue! It is the symptom of anaerobic glycolysis responding to the bodies need for energy. Lactic acid and lactate are not to blame for muscle soreness either, since blood lactate levels return to normal within an hour or so of intense training (4).
Lactic acid (pyruvate + H+) + Na+ = Lactate (pyruvate + Na+) + H+
NADH + H+ = NAD+
Lactate Energy Theories
The only way for lactic acid to leave working muscle cells is to be converted to lactate. Some believe lactate can be used by the body as another source for regenerating ATP. I'm sure you're praying this is true when prowler work comes up, and there are several theories as to if and how the body processes lactate for energy. The simplest of which is that lactate is diffused and transported in the bloodstream to areas in direct need of energy, for example the heart and brain. This theory also states the lactate can be transported and reconverted to glycogen in the liver through the Cori cycle (4).
Another method thought for using lactate as energy is when it is transported directly to nonworking muscle fibers adjacent to the working fibers that created lactate. This transport has been found to occur mostly from fast twitch to slow twitch fibers in the same muscle group. The lactate is produced rapidly in the fast twitch and sent to the slow twitch that have a higher pH and can metabolize this to ATP for later use (3).
A more intriguing theory is that lactate can be used to provide energy to the working muscle fibers where it was produced. One study (Brooks, et al. 2005) showed that lactate can be shuttled into the mitochondria of the muscle fibers to regenerate ATP in the cell it was created (1). The study theorized that lactate entered the mitochondria in the cell it was produced and then reconverted to pyruvate. This pyruvate then would be metabolized by CO2 to regenerate ATP in the same fiber using Intracellular Lactate Shuttles (3). This theory has not been universally accepted, but the existence of lactate transporters has been well documented. So expect to see some more studies done to challenge or support this theory in the near future.
Lactic Acid in Connection with Growth Hormone
Quick review, lactic acid is a byproduct of anaerobic glycolysis, the breakdown of glycogen for energy. Where things get interesting is when examining lactic acid as it relates to McArdle’s Disease. This disease does not allow an individual’s body to breakdown glycogen for energy. If the body cannot breakdown glycogen, then it is unable to create lactic acid or lactate. McArdle’s Disease has prompted a different research approach to lactic acid: can we find out what the effects of lactic acid are by testing individuals who can’t produce it?
A study (Godfrey, et al., 2009) hypothesized that increased blood lactate concentration is a primary stimulus for exercise induced growth hormone response (EIGR). Eleven patients with McArdle’s disease were used for the study. They were the perfect models to test this hypothesis since they were unable to produce lactate in response to exercise. 9 of the 11 participant’s blood lactate levels remained at resting levels after exposure to anaerobic exercise and failed to show an exercise induced increase in growth hormone. The study concluded that lactate could play a major role in EIGR.
Implications on training
For years the training approach for lactic acid has been to improve physiological mechanisms that reduce production and accumulation from an endurance perspective. The belief lactic acid was responsible for fatigue and other negative effects, has long been propagated in explosive sports utilizing the field/rink/court. Training a Power Athlete's metabolic systems like an endurance athlete will have negative effects on what makes them 'Power' Athletes. New developments coming out consistently support the hypothesis that lactic acid is not responsible for acidosis, and it could possibly used for energy and stimulating EIGR. There has been an evolution in training practices for explosive sport athletes, as well as the introduction of the Sport of Fitness, that have given us more reason to believe that lactic acid can be used as a tool.
Part 2 will dive into how an understanding of Lactic Acid can be a valuable tool in itself to apply to developing strength and conditioning programs. The application of that tool comes down to one thing....
What Are You Training For?
(1) Brooks, G.A., T.D. Fahey, and K.M. Baldwin. (2005). Exercise Physiology: Human Bioenergetic and Its Application. New York, NY: McGraw-Hill.
(2) Godfrey, R.D., et al. (2009). The Role of Lactate in the Exercise-Induced Human Growth Hormone Response: Evidence From McArdle Disease. British Journal of Sports Medicine. July 2009; 43(7):521-5.
(3) Maglishco, Ernest W. (2012). Does Lactic Acid Cause Muscular Fatigue? Journal Of The International Society of Swimming Coaching. March 2012, Vol 2, Issue 2.
(4) Verkhoshansky, Y., & Siff, M. (2009). Supertraing: 6th Edition. Rome: Ultimate Athlete Concepts.
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Former collegiate lacrosse defensive midfielder, 4-year letter winner and 3-year team captain. Coached strength and conditioning collegiately with Georgetown University football, Men's and Women's lacrosse and Women's Crew, as well with the University of Texas at Austin's football program. Apprenticed under Raphael Ruiz of 1-FortyFour-1 studying proper implementation of science based, performance driven training systems. Head coached CrossFit Dupont's program for two years in Washington D.C. Received a Master's in Health Promotion Management from Marymount University in 2010, and has been a coach for Power Athlete since October, 2012.