What Causes Lactic Acid To Build Up In Muscle Cells?

Lactic acid, that notorious ‘burn' you feel during an intense workout, builds up in muscle cells due to a lack of oxygen, leading them to produce energy anaerobically.

While this acid isn't the culprit for post-workout soreness, it does play a central role in that familiar fiery sensation. Curious about the science behind it? Dive in for a detailed look!

Understanding Muscle Respiration

Every time you move, whether it's lifting weights, walking, or even just breathing, your muscles are hard at work.

But how do these muscles get the energy to function?

It's through a process called muscle respiration. Let's delve deep into the world of muscle respiration and uncover its intricacies.

Differentiating between Aerobic and Anaerobic Respiration

Muscle respiration can be broadly categorized into two types: aerobic and anaerobic.

  1. Aerobic Respiration:
    • Definition: This is the process where cells produce energy by breaking down glucose in the presence of oxygen.
    • Energy Production: It yields a considerable amount of energy, specifically 36-38 molecules of ATP (Adenosine Triphosphate) from one molecule of glucose.
    • By-products: Carbon dioxide and water are produced as waste, which we breathe out.
    • Duration: This process supports prolonged activities. For example, when you're on a long run or cycling, your muscles are primarily using aerobic respiration.
  2. Anaerobic Respiration:
    • Definition: This occurs when there's not enough oxygen available for the muscle cells to produce energy.
    • Energy Production: Less efficient than its aerobic counterpart, it generates just 2 molecules of ATP from one molecule of glucose.
    • By-products: Lactic acid is a primary by-product, which can lead to the burning sensation in muscles during high-intensity activities.
    • Duration: Supports short bursts of high-intensity activities. Think of sprinting or heavy weight lifting where you exert a lot of force in a short duration.

The Energy Demands of Muscles During Activity

Our muscles are like engines, and just like how engines need fuel to run, muscles need energy to function.

The type and intensity of the activity determine the energy demand.

  1. Resting State:
    • When you're at rest or doing low-intensity activities, your muscles aren't demanding a lot of energy. Aerobic respiration is the predominant process here, ensuring that cells have a steady supply of ATP.
  2. Moderate Activities:
    • Activities like walking or light jogging require more energy than when you're at rest. Still, the muscles can get enough oxygen to continue producing ATP through aerobic respiration.
  3. High-intensity Activities:
    • As the intensity ramps up, like in sprinting or powerlifting, the energy demand spikes. The body needs quick energy, faster than aerobic respiration can provide. This is when anaerobic respiration kicks in, delivering rapid bursts of energy.
  4. Transitioning Between Activities:
    • When you switch from a state of rest to sudden activity, or vice versa, your body adapts its energy production methods. It might take a minute or two for your breathing and heart rate to catch up with the demands of a sudden sprint, causing temporary reliance on anaerobic respiration.
  5. Fueling the Muscles:
    • Glycogen stored in muscles acts as the primary source of glucose for energy production. As you engage in activities, the glycogen reserves are used up. Once depleted, your body taps into other energy reserves, like fats.
  6. Post-exercise Oxygen Consumption:
    • After engaging in high-intensity activities, you might be panting, trying to catch your breath. This is your body's way of repaying the “oxygen debt.” It's processing the accumulated lactic acid and restoring the body to its resting state.

The Science Behind Lactic Acid Production

Whenever you feel that familiar burn in your muscles during an intense workout, lactic acid is often at play.

But what triggers its production, and why does it happen? Let's delve into the biochemistry of lactic acid generation in our muscles.

The Role of Glucose in Muscle Cells

Glucose, a type of sugar, acts as the primary fuel for our cells. Our bodies derive glucose from the food we eat, especially carbohydrates.

Once digested, glucose enters the bloodstream and is then taken up by various cells, including muscle cells, to meet their energy needs.

Inside the muscle cells, glucose undergoes a series of chemical reactions to produce energy.

This series of reactions, collectively known as glycolysis, breaks down glucose and releases energy, which is then stored in molecules called ATP (Adenosine Triphosphate).

ATP is like the energy currency of the cell, which it uses to power various functions.

Now, the end product of glycolysis can vary based on the availability of oxygen.

When there's ample oxygen, the end product is typically processed further in the mitochondria, the cell's powerhouse.

However, in the absence of sufficient oxygen, the cell adopts a different strategy, leading to the production of lactic acid.

How Muscles Create Energy When Oxygen Supply is Low

When you engage in intense physical activity, your muscles work harder and demand more energy than usual.

As the demand for energy increases, the muscles need more oxygen to produce this energy aerobically.

However, there may be times, especially during strenuous exercise, when the supply of oxygen to the muscles can't keep up with their demand.

In such situations, the muscles resort to anaerobic respiration to meet their energy needs.

In anaerobic conditions, the glucose still undergoes glycolysis, but instead of moving to the next stages of aerobic respiration, the end product of glycolysis (a molecule called pyruvate) gets converted into lactic acid.

This conversion is a way for the cells to regenerate a crucial molecule called NAD+, which is essential for glycolysis to continue.

By turning pyruvate into lactic acid, cells ensure that they can keep producing ATP, even if it's at a lower rate than aerobic respiration.

However, there's a trade-off. The accumulation of lactic acid in the muscles leads to a decrease in muscle pH, resulting in the sensation of muscle burn.

This acidic environment inhibits the enzymes involved in energy production and can temporarily hinder muscle contractions, often serving as a natural signal for us to slow down or pause.

Effects of Lactic Acid Accumulation

That fiery sensation in your muscles after a few more reps or that last sprint isn't just in your head—it's a result of lactic acid accumulation.

But beyond that burning feeling, what's truly happening inside our muscles? Let's delve into the repercussions of lactic acid buildup.

The Impact on Muscle pH: What Does Acidosis Mean?

When we talk about the pH level, we're referring to the measure of acidity or alkalinity in a solution.

The human body tightly regulates its pH to maintain optimal functioning, especially in the blood and muscle cells.

Muscle cells ideally function in a slightly alkaline environment, but as lactic acid accumulates during intense physical activity, this balance is disrupted.

Lactic acid dissociates into lactate and hydrogen ions in the muscle cells.

It's the hydrogen ions that cause the drop in pH, leading to a more acidic environment.

This state of increased acidity in the muscles is termed “acidosis.”

Acidosis affects muscle performance in several ways.

First, it can interfere with the ability of muscles to contract, diminishing their force and endurance.

Second, the enzymes that play a crucial role in energy production and muscle contractions can be less effective in an acidic environment.

Lastly, an acidic muscle environment can also slow down the energy-producing reactions necessary for both aerobic and anaerobic respiration.

Decoding the Burning Sensation During Intense Workouts

We've all felt it—that intense burn when we're pushing our limits, whether it's during the last stretch of a run, the final set of squats, or the concluding minutes of a high-intensity interval training (HIIT) session. But what's the science behind this sensation?

The burning sensation in muscles during rigorous exercise isn't solely due to lactic acid, even though it's a popular belief.

As mentioned, lactic acid splits into lactate and hydrogen ions.

The rising concentration of hydrogen ions in the muscles contributes to the drop in pH, resulting in the acidic environment we discussed.

This acidic setting not only inhibits the enzymes but also affects the nerves in the muscle tissue.

The nerves respond to these changes by sending signals to the brain, which interprets this as a burning sensation.

This feeling is essentially a protective mechanism—it's the body's way of telling you to ease up or take a break, ensuring you don't push yourself to a point of severe muscle damage.

Debunking Myths

Lactic acid is often the talk of the town, especially in fitness circles, and with its popularity come a few myths that need debunking.

Some notions have been so deeply rooted that they've become almost universally accepted truths. Let's set the record straight on some of these misconceptions.

The Misconception Between Lactic Acid and Post-Workout Muscle Soreness

A common belief held by many is that lactic acid is the culprit behind the muscle soreness felt a day or two after an intense workout.

This sensation, known as delayed onset muscle soreness (DOMS), is often wrongly attributed to lactic acid buildup.

In reality, lactic acid levels in the muscles return to their normal state within an hour or so after exercise, far too quickly to be responsible for the soreness felt days later.

DOMS is believed to be a result of microscopic damage to muscle fibers during intense physical activity, especially activities that the body isn't accustomed to.

When we push our muscles harder than usual or introduce them to new exercises, tiny tears occur in the muscle fibers.

The body then responds by repairing and strengthening these fibers, a process that can lead to soreness and stiffness.

While lactic acid may cause a burning sensation in muscles during exercise due to the reasons discussed earlier, its association with post-exercise soreness is, in essence, a myth.

Other Factors Contributing to Muscle Discomfort Post-Exercise

While lactic acid may have been exonerated from causing DOMS, other factors come into play when considering the discomfort we feel after a strenuous workout.

One significant factor is the inflammation that sets in after exercise-induced muscle damage.

The body recognizes the tiny tears in the muscle fibers as injuries and sends in inflammatory cells, growth factors, and nutrients to begin the repair process.

This inflammation can lead to muscle swelling, pain, and a temporary loss of strength.

Another contributing factor is muscle spasms. After an intense workout, some muscle fibers may remain contracted, leading to spasms.

These contractions can cause discomfort and a feeling of tightness in the affected muscles.

Additionally, the buildup of metabolic waste products in the muscles can play a role.

While lactic acid clears out quickly, other by-products of muscle metabolism, like inorganic phosphate and ions, can accumulate and contribute to muscle discomfort.

Lastly, connective tissue damage and stress on the muscle-tendon junctions can also be a source of pain.

When muscles are overloaded, the connective tissue and tendons bear the brunt of this excess force, leading to discomfort.

Methods to Manage Lactic Acid Buildup

There's no escaping it: if you're working out intensely, you're likely to feel the effects of lactic acid.

But, rather than viewing it as a nemesis, consider it a sign that you're challenging your muscles.

Still, too much discomfort can hinder performance, so let's explore strategies to manage and mitigate lactic acid accumulation.

The Significance of Warm-Ups and Cool-Downs

Engaging in a thorough warm-up before diving into the main workout is akin to prepping a car engine on a cold morning; it ensures everything runs smoothly.

Warming up gradually increases heart rate, blood flow, and oxygen supply to the muscles.

As blood flow improves, muscles are better primed to handle the increased demands of a workout, potentially delaying the onset of anaerobic respiration and lactic acid production.

Warm-up exercises, like light jogging, dynamic stretching, or performing a lighter version of your main workout, serve to prepare your muscles, tendons, and joints for more intense activity.

When muscles are adequately warmed up, they become more efficient in using available oxygen, thereby relying less on anaerobic processes.

Cooling down after a workout, on the other hand, is equally essential.

A cool-down, which typically involves lighter activity and stretching, aids in gradually bringing the heart rate down and promoting recovery.

Moreover, it facilitates the removal of metabolic waste, including lactic acid, from the muscles.

By enhancing blood flow, cool-downs assist in transporting lactic acid to the liver, where it can be converted back into glucose or glycogen.

Breathing Techniques and Their Impact on Oxygen Supply

Breathing might seem like the most natural thing in the world, but when it comes to exercise, the way you breathe can profoundly impact your performance and the accumulation of lactic acid.

Proper breathing ensures that your muscles receive a consistent supply of oxygen.

Remember, lactic acid builds up when muscles resort to anaerobic respiration due to a lack of oxygen.

By optimizing oxygen intake and delivery, you can potentially delay the switch to anaerobic processes.

Deep, diaphragmatic breathing, as opposed to shallow chest breathing, allows for a fuller intake of oxygen with each breath.

When you breathe deeply from the diaphragm, more air reaches the lower parts of the lungs where a vast majority of blood vessels reside.

This means more oxygen is taken up by the blood and transported to the working muscles.

Additionally, rhythmic or paced breathing can also be beneficial.

This involves synchronizing your breath with your movements.

For example, in running, one might take a breath every two or three strides.

This rhythmic approach ensures a steady supply of oxygen to the muscles.

Moreover, practicing breathing exercises outside of your workouts, like those found in yoga or meditation, can improve lung capacity and efficiency.

Over time, this can lead to better oxygen utilization during physical activity.


As we push our bodies to their limits, understanding the nuances of lactic acid and its effects is crucial.

While it's a natural byproduct of intense exercise, with the right knowledge and strategies, we can manage its buildup, ensuring our workouts are both effective and comfortable.

After all, knowledge is power, and in this case, it's the power to optimize our physical endeavors.