Lactic Acid, it’s likely that most of us have heard of it—whether it was from a 1980’s Richard Simmons aerobics tape or from your personal trainer last week. The fitness industry has tossed this term around for years to describe the infamous “burn” during a workout. We were told that during intense exercise, lactic acid builds up in our muscles causing us to experience fatigue. Just this morning, I took a cycling class and the instructor’s exact words were, “you should be feeling that lactic acid build up, just 1 more minute on this hill (to push harder). If you’re not feeling the burn, add more resistance on your bike!” Most of us will just nod our heads and suffer through the workout, but research has presented an entirely different story behind lactic acid. As a fitness professional who has heard it all, I feel as though I need to set the story straight.
The confusion around lactic acid started in the 1920’s when scientists observed higher levels of lactic acid in a frog’s blood and muscle tissue when oxygen was completely absent (which, unless you are dead, doesn’t happen in the human body). This eventually led to the idea that lactate is only produced during really intense exercise (i.e when you’re breathing heavily and working as hard as you can). Given that lactate levels and exercise intensity were highly correlated, it’s no wonder that this assumption of cause and effect caught on so fast. Here’s the catch that probed one particular scientist to dig a little deeper into the dynamic of lactic acid build up and fatigue: lactic acid forms during the presence of oxygen too—in fact, absence of oxygen is not really the basis of lactic acid formation at all.
What is lactic acid, anyway? Let’s run through a scenario to help better understand its purpose. Your feet hit the pavement and you’re off on your morning run…
If you are an average healthy individual, your muscles have stored up a sufficient amount of carbohydrates in the form of glycogen from the meal you had the night before. Within minutes of starting your run, your muscles will start to break down the glycogen into smaller energy units of glucose. Cells then have the ability to break glucose down even further and convert it into two forms: pyruvate and lactate (lactic acid does not form in the body, but this is really just an issue of semantics). If you’re exercising at a moderate level (something you can sustain for more than 20 minutes) you’re breathing in through the nose and out through the mouth (so there’s plenty of oxygen present), pyruvate will readily enter into compartments inside the muscle cells called mitochondria—this is where the magic happens and ATP (energy required for all cellular work) is formed. Here’s where you might be wondering what determines whether glucose yields lactate or pyruvate, and how much?
Your cells aren’t just producing lactate or just producing pyruvate at a given time—the human body is constantly seeking equilibrium, so there will always be some lactate being produced. At the start of exercising, the ratio of pyruvate to lactate that’s being formed is shifted more towards pyruvate. By in large, this ratio is really dependent on how many mitochondria in your muscles are available to take in the pyruvate, and how fast that first step of glucose break-down is occurring. You can imagine, that as your exercise intensity increases, your muscles are going to demand more energy, and in response, this first step will start to turn over faster.
Returning to your feet hitting the pavement—it’s about 10 minutes into your moderate run and the energy compartments inside your muscle (mitochondria) are beginning to reach capacity of how much pyruvate they can take up and turn over into ATP.
Here’s where the lactate comes in. When the mitochondria can no longer take up pyruvate, they will start to produce more lactate to compensate (i.e. the ratio shifts in the other direction). Again, this explains why sports professionals and trainers alike started calling lactate (or inaccurately, lactic acid) the culprit for muscle fatigue. Lactate levels increase as performance coincidently drops off. However, studies have shown that during intense exercise bouts, lactate itself in the muscles does not significantly impact the ability of the muscle to contract and produce force.
So it’s not just when you’re exerting your muscles at full force that lactate starts to appear. More importantly when it does appear, lactate is not the dead-end metabolite we all thought it was. Muscles actually use lactate for energy during both moderate and hard exercise. With the help of a transport molecule, the mitochondria have the ability to take up lactate, convert it back to pyruvate once inside, and turn it into the ATP your muscles need for work. Interestingly, as your fitness improves, your muscles start creating more mitochondria. More mitochondria, means you can take up even more lactate and use it for energy. After a few months of a cardio routine, your blood lactate levels will actually decrease for a given exercise intensity—proving that you are taking up lactate more efficiently.
Point of the story: lactate is fuel, not waste.
You may be wondering if it’s not the lactate that causes muscle fatigue, then what is it?
This question is still being tossed around in the literature. However there is one idea that may prove to be the culprit behind muscle fatigue in light of the processes that I previously covered—the breakdown of glucose to pyruvate and lactate, the entering into the mitochondria, and then the conversion to ATP. One thing that can significantly effect the acidity inside your muscle tissue (or in any solution) is hydrogen. The more hydrogen the lower the pH (more acidic). During exercise, hydrogen is released in abundance as a result of breaking down fuels. Experiments have shown that the largest decreases in pH (muscles becoming more acidic) are during short bouts of intense, maximal effort that last anywhere from 1-10 minutes. In line with this idea, muscle force is also seen to decline during these extreme states of acidosis in the muscle. Whether acidosis is the cause of muscle fatigue or just somehow related, is something that is still being debated, but it seems to be the most plausible mechanism at this time.
Does our new knowledge of lactate’s role change anything about how athletes should train for the future?
Not really, but it does allow athletes to think about training intensity a little differently. As we’ve found out, the body goes through some pretty significant adaptations after training—one of them is being able to use lactate as a fuel more efficiently. Thus, why training at higher intensities (when lactate does start to rise), has proven to be most effective for performance. While the methods of training might not change, the explanations behind them do. As a personal trainer, it is important to stay current with the research, so I can accurately educate my clients. Unfortunately, going off sound bites and popular beliefs is often the default for many fitness professionals. Next time you experience a really tough workout, appreciate the lactate for keeping you going. As for the “burn”…Burn Baby Burn!
George Brooks, et al.(2005). Exercise Physiology: Human Bioenergetics and Its Applications, 4th Edition. Chpt 5 pp. (59-92).
Anita M Rivera-Brown, et al. (2012). Principles of Exercise Physiology: Responses to Acute Exercise and Long-term Adaptations to Training. Exercise and Sports for Health Promotion, Disease, and Disability. doi:20.1016/j.pmrj.2012.10.007
Cairns SP. (2006) Lactic acid and exercise performance: culprit or friend? Sports Medicine. http://www.ncbi.nlm.nih.gov/pubmed/16573355
Robert, A. Robergs, et al. Biochemistry of exercise-induced metabolic acidosis. Am J. Physiol Regul Comp Phyisol. doi: 10.1152/ajpregu.00114.2004
Rogerio Santos de Oliveira Cruz, et al. (2012). Intracellular Shuttle: The lactate Aerobic Metabolism. The Scientific World Journal. doi: 10.1100/2012/420984
LA, Messonier, et al. (2013) Lactate Kinetics at the lactate threshold in trained and untrained men.Journal of Applied Physiology. doi:10.1152/japplphysiol.00043.2013