If you’re a runner, then you’ve no doubt heard about lactic acid. This substance is often (incorrectly) blamed for the muscle soreness that runners experience after a long run. But what is lactic acid, and what role does it play in running, fatigue, and soreness? Let’s dispel some of the myths about this substance.
What is lactic acid?
Lactic acid is a compound that is produced when the body breaks down carbohydrates for energy production. However, it is quickly broken down into lactate and a hydrogen ion. (As such, blood tests done on athletes measure levels of blood lactate – not blood lactic acid).
While it’s gotten a bad reputation in the past – lactate is actually helpful. It plays several roles in the body, including as an energy source for certain cells (like those in the brain) and acting as a signaling molecule (helping translate messages between different parts of our cells).
What is lactate threshold for runners?
Once your body reaches a certain level of intensity during an exercise, lactate levels start to rise at a faster rate. The pace or intensity where this occurs is often called “lactate threshold” in the running community.
The specific pace or heart rate where you hit your lactate threshold will be different than other runners; it’s highly individualized based on genetics as well as your personal fitness level.
While doing a true lactate threshold test would require an in-lab experience, it’s relatively simple to get an estimation by doing a lactate threshold field test. One simple version involves running for 30 minutes at the best pace you can sustain consistently throughout that time. Your average heart rate during the last 20 minutes of the test is what’s called your lactate threshold heart rate.
Knowing this heart rate can help with structuring zone-based heart rate training plans.
Many people think too many high-intensity runs are “bad” because of lactate accumulation. In reality, though, it’s other aspects of working too often at a high intensity that cause negative consequences. Lactate itself is likely more innocuous than you might realize.
(Of note, even though I’ll refer to this in the context of running, it’s important to keep in mind that lactate accumulation can also occur with strength training.)
Let’s look at two common questions that are often asked…
Does lactic acid cause muscle soreness after a workout?
One of the most common myths about lactic acid is that it causes delayed onset muscle soreness. This is not true.
The pain that you feel the day after a workout is actually caused by micro-injuries to the muscles. These injuries occur as a result of the strenuous activity placing stress on the muscle fibers – not because of lactic acid.
Certain exercises are more likely to elicit DOMS than others. Eccentric contractions – contractions that occur as the muscle lengthens, like downhill running – increase the risk of DOMS.
Does lactic acid cause muscle fatigue during a workout?
Contrary to popular belief, the answer to this is not clear cut. We actually know now that fatigue is not due to lactic acid alone (if at all).
It’s been a long-held belief that lactic acid (or perhaps more accurately, lactate plus a hydrogen ion) was responsible for muscle fatigue by causing acidosis in the muscles. It sounds simple: if you end up with lactate and a hydrogen ion, that would increase hydrogen ions in the muscle, making the muscle acidic – right?
Not so fast. Some research suggests muscle acidosis is more likely caused by other metabolic processes. The main metabolic processes theorized to cause acidosis are related to steps in glycolysis and the breakdown of ATP to produce energy.
I know this is a little science, so let me try to break down this theory:
When you are running at a comfortable steady state, the body is able to create energy in the mitochondria of your cells via aerobic (oxidative) energy production. This system allows us to exercise at moderate intensities for long periods of time. These metabolic processes are not thought to cause proton accumulation in the cell – hence, no acidosis.
However, let’s say you’re at the end of a 5K race. You’re picking up the pace at the end to try to PR. In these situations, when the body works at an intensity above steady state exercise, you start delving into another energy production method called anaerobic metabolism. During these reactions to create energy, extra protons are released. When too many protons accumulate, it can lead to acidosis in the muscles.
Acidosis is thought to cause fatigue by creating an environment where the muscles cannot contract as effectively and there is reduced force production.
Under this theory, then, researchers believe the timing of acidosis simply falls in line with the timing of lactate accumulation – but that lactate isn’t the cause of the acidosis. Interestingly, they postulate that lactate may actually help the muscles delay fatigue during this time. Under their biochemical constructs, they believe the lactate dehydrogenase reaction may serve to consume protons and thus possibly delay developing metabolic acidosis.
It’s important to note that this is one theory. Other researchers do not agree with this and maintain that lactic acid production is still a primary factor in muscular acidosis.
However, a recent 2021 study attempted to quantify the impact of the many biochemical processes that occur during energy production. They found that the release of hydrogen ions (leading to acidity) was approximately 4 times as large as would be expected under the lactic acid theory. This suggests other processes are at play in acidosis.
So what’s the truth? We don’t know all the facts yet – but it clearly seems that lactic acid is not the only cause of muscle acidosis (if it’s a cause at all).
Plus, there’s an additional wrench thrown into the mix – studies suggest acidosis is not the only factor that may limit muscular contractions and force.
For example, one study looked at the impact of sodium bicarbonate (a buffering agent) on running performance. Interestingly, they found no impact on time to exhaustion with high intensity running – and no severe muscle acidosis at the end of exercise. If muscle acidosis would have been the primary factor causing exhaustion, one would think that the time to exhaustion would have increased with a buffering agent. Instead, it’s likely that another factor (mental or physical) led to exhaustion in this case.
Similarly, several human studies have suggested that subsequent sprint performance has been limited not by acidosis, but by phosphocreatine availability (source). And other research suggests that different factors may play a role in muscle fatigue too, such as inorganic phosphate accumulation, neural contributors, or exercise induced oxidative stress.
TLDR: Muscle fatigue is about much more than just lactic acid.
Is paying attention to lactate useless?
Because rapidly rising lactate levels correlate with a heavier reliance on anaerobic energy production, they are an indicator that the body is working above its aerobic means. Levels may correlate with increased risk of muscle acidosis or accumulation of other metabolic byproducts that can affect fatigue.
As such, if a distance runner was able to monitor their lactate levels, they would get some insightful data that could lead to slowing of the pace and better building an aerobic base.
Of course, monitoring of blood lactate levels is pretty impractical outside of lab settings.
But by implementing a lactate threshold field test, or by simply better paying attention to respiration and ability to speak at a given pace, runners can ensure they’re truly working below lactate threshold on their easy conversational runs. This can maximize training adaptations for distance runners, where building aerobic capacity is key to performance.
Speed work above that lactate threshold is essential to many training plans. This helps you train to run fast. But it’s important to avoid too much speedwork in a plan – not because of worries about lactate accumulation, but because of increased risk of injury and overtraining issues.
The Bottom Line
Lactic acid – more aptly described as lactate and a hydrogen ion – play an important role in running through several mechanisms like energy production and signaling. This substance is not responsible for delayed onset muscle soreness. While it may play some role in muscle acidosis, new research suggests it may not be the primary cause.
As a runner, though, understanding lactate threshold can help guide a proper training plan – not due to the lactate accumulation itself, but by being a proxy measure for the crossover from aerobic to anaerobic training.
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