Understanding Lactic Acid and Its Role in Muscle Fatigue

If you've ever pushed yourself during a tough workout—maybe sprinted up a hill, lifted weights in a new way, or got back on a bike after a long time off—you’ve probably felt that burning sensation in your muscles and some soreness the next day. And chances are, at some point, you have connected those sensations with lactic acid. 

Well, you are not alone! 

For decades, lactic acid has been blamed for everything from muscle fatigue to post-workout soreness. Until I did some research, I definitely believed that a buildup of lactic acid was the cause of my sore muscles. But now I know that that is not the whole story and is not even close to accurate!

Let’s dive into what lactic acid actually is, how it works in the body, and what role it plays in muscle fatigue.

What is Lactic Acid?

First things first—lactic acid is a byproduct of anaerobic metabolism. Anaerobic metabolism occurs when our muscles need energy fast (like during intense exercise), and our bodies can’t get oxygen to them quickly enough. Anaerobic metabolism is the backup system. This system breaks down glucose (sugar) for energy without using oxygen. The result is energy to fuel the muscles (which is the goal) and the byproducts lactate (also referred to as lactic acid) and hydrogen ions. Interestingly, it’s actually the buildup of the hydrogen ions (which make our muscles more acidic), not the lactate, that contributes to the acidity in our muscles and leads to muscle fatigue and that burning feeling.

And, to make this more interesting, it turns out that lactic acid might not only NOT cause muscle fatigue, but it might actually delay muscle fatigue!

Let’s unpack that.

Lactic Acid in Muscles

When our bodies ramp up their energy demands and switch to anaerobic metabolism (the process of making energy without oxygen), lactate production increases. Like we mentioned before, this process does release hydrogen ions, which can lower pH in our muscles and make them more acidic, thus leading to that feeling of muscle burn. However, (and amazingly!) our bodies can actually use lactate as a fuel source. Our hearts, brains, and even slow-twitch muscle fibers can gobble up that excess lactate and use it to make more energy! So, lactate is both a byproduct of anaerobic metabolism and an extra fuel source! That’s pretty cool - lactate is actually part of our bodies’ solution to keep going when the going gets tough. And, the increased amount of lactate in our systems may actually signal our systems to adapt to the stress of the physical exertion, thus improving our fitness levels. 

See! Lactate isn’t that pesky thing that makes workouts unpleasant. It’s actually a pretty savvy biological adaptation that helps to keep us moving when oxygen is in short supply. Our bodies are amazing!

Muscle Fatigue

Now that we understand that lactic acid is not responsible for muscle fatigue, the next obvious question is, what causes muscle fatigue? 

And, the answer is…it’s multi-faceted.There are several things that are happening in our bodies when we are pushing our muscles to work hard that can make it feel like our muscles are tired. These include:

1. An accumulation of hydrogen (remember - when glucose is used for fuel without oxygen, hydrogen is a byproduct). When hydrogen accumulates, the acidity in the muscles is increased and that can interfere with the enzymes that are involved with muscle contractions, thus making it harder to keep pushing. 

2. The depletion of ATP (the primary energy source for muscles). As we burn through ATP, our muscles slow down. 

3. A decrease in the rate at which calcium is released into the muscles which can make it harder for our muscles to contract. 

4. Mental fatigue. When our central nervous system senses that we are working too hard, it can reduce the rate at which our muscles are firing, leading to muscle fatigue.
   
   

Lactic Acid and Exercise

What about all of the lactate and extra hydrogen ions that are released when we burn glucose without oxygen? Well, they get cleared from our bodies pretty quickly. Studies show that lactate levels return to baseline within about 30–60 minutes after exercise, especially if we keep moving. An “active recovery” like walking or stretching is often recommended after intense sessions—it helps flush out those byproducts and gets us feeling better faster.

And, that soreness and tight muscles feeling we get later in the day, or even the next day after an intense workout (called delayed-onset muscle soreness)? That’s not caused by lactic acid either! That soreness is actually from tiny microtears in our muscle fibers, not from lactate buildup. It’s part of the normal healing and adaptation process. So if you’re hobbling around the day after lifting heavy, don’t blame lactic acid—blame your dedication and your body’s natural healing process that includes inflammation and pain!

Lactic Acid Buildup

Can you see how, even though the term “lactic acid” has been used for decades, it’s actually not an accurate term. Although I admit that it might still be easier to say “lactic acid buildup” than “accumulation of lactate and hydrogen ions from anaerobic glycolysis” when talking with our gym buddies!

But as science evolves, maybe our understanding can as well. And the truth is, the more we learn about what our bodies are designed to do, the more respect we can have for them. Lactate is not a toxin—it’s a valuable player in our bodies’ energy systems. But, whether or not you are interested in the actual cause of your muscle fatigue and soreness, just keep working! Challenging our bodies physically and building endurance and strength is important for our bodies, our minds, and our longevity

Now go crush that workout!