Lactic Acid and Lactate: Understanding the Threshold

Lactate has often been blamed when your legs feel heavy, but is it really at fault? This is a two-part article, and in this first part, we explore what lactate and "lactic acid" actually are, how lactate is formed, and why it is frequently misunderstood in training discussions. The idea is simple: lactate is not just a sign that things are getting tough—it's also a fuel the body can use and move between tissues. What primarily affects performance when you exert yourself is the increased acidity in the muscle, associated with hydrogen ions. With this understanding, it's easier to comprehend threshold sessions and how lactate can be practically applied, but we will delve into that in part 2.

There's also something known as lactic acid, but it's not present in the body for more than a few seconds in its undivided form. The pKa of lactic acid is about 3.86, which means that at a physiological pH (around 7.4), lactic acid is practically completely dissociated into lactate and hydrogen ions. So: the "lactic acid" mentioned on the running track is actually hydrogen ions (H+) that lower pH, and lactate is a more stable ion that the body actually uses — as noted by some researchers.

The designation for hydrogen (H+) is what determines pH. When measuring pH, you're essentially measuring the amount of hydrogen ions: more ions = a more acidic environment and lower pH (it's a negative scale, so think opposite first — then you'll get used to it).

Since lactate is a stable ion and easy to measure, it early on shouldered the blame as "the bad guy." Researchers observed that when people exerted more effort, lactate levels increased — and performance decreased. The correlation quickly became: more lactate = worse. The problem was that lactate in many cases was the helpful partner trying to buffer pH and serve as fuel.

Lactate is primarily formed in our more explosive muscle fibers (type II) that have fewer blood vessels, lower oxygen transport, and fewer mitochondria. The lactate produced can then move over to fibers with better oxidative capacity (usually type I fibers with plenty of mitochondria and blood supply). This is what researchers describe as the lactate shuttle (Cell-to-Cell and Intracellular Lactate Shuttle).

In muscle cells, about 75 percent of the produced lactate is oxidized, forming energy. The heart muscle is particularly efficient at using lactate — it's like a blast furnace and can run on just about any fuel you can imagine. The remaining approximately 25 percent is transported to the liver and converted to glucose via gluconeogenesis, which helps regulate blood sugar during activity.

The lactate can thus be transported to the liver and converted back to glucose via the Cori cycle. This process serves as a kind of backup to continue providing energy when oxygen intake isn't sufficient and partly helps maintain blood sugar levels during high-intensity periods (source). As lactate forms, hydrogen ions are released, lowering the pH in the working cell — and that’s the acidic feeling in your muscles.

In summary: it's not the lactate that's the villain. When you push so hard that oxygen isn't enough, the body starts using its reserve solutions — lactate production increases — but the price is increased free hydrogen ions that ultimately impair performance. In our article on bicarbonate, you can read more about how we can manage these free hydrogen ions to push harder during intervals and threshold sessions.

At rest, we have approximately 1 mmol of lactate in the blood—there's always some lactate present in the body. When you exert yourself intensely, the amount of lactate increases (and thus the number of hydrogen ions too), and well-trained individuals can reach levels up to 15 mmol of lactate in the blood.

When exertion is so intense that the production of hydrogen ions exceeds the body's ability to transport them away, the cell's pH balance is disrupted. This is when we cross what is known as the lactate threshold—or simply the threshold in everyday terms. For those who are untrained, the first lactate threshold occurs at around 50–55 percent of VO2max. In well-trained athletes, it's around 75 percent of VO2max (about 80 percent of maximum heart rate).

It's important to remember that lactate in the blood is just an indicator of what's happening in the muscle—and it's an indirect marker for the amount of hydrogen ions. Not all the lactate produced makes it into the bloodstream; some is oxidized directly in the muscle where it is produced. Therefore, comparing the blood lactate values of two people doesn't always provide a fair assessment of their capacity: Person A might excel at using lactate locally as fuel while Person B might not. However, lactate testing remains a valuable tool for tracking your own progress over time.

Raising your lactate threshold is crucial: it raises your limit and allows you to train and compete harder before the buildup of hydrogen ions starts to reduce performance. What you develop through threshold sessions and intervals is primarily your body's ability to manage oxygen so that the activity stays aerobic — that's when everything works optimally. Threshold sessions enhance oxygen transport through the development of more mitochondria and capillaries. At the same time, the amount of enzymes and "carrier molecules" that regulate pH, energy balance, and other factors in the cells increases. In short: you become better at delivering oxygen to the cells even at higher intensities, which helps you stay below the threshold longer.

It's also worth mentioning that intervals designed to increase VO2max generally produce similar results. By improving your ability to utilize oxygen (your VO2max), you can train and compete harder without the cell environment becoming anaerobic. While it's not exactly the same as specific threshold sessions, the result — a higher capacity for sustainable intensity — is often similar.

This was part 1 of 2 in our article series about lactic acid. Here, we covered the basics — what lactate is, how it is formed, and why it is often misunderstood. To comprehend practical applications in the next part, this foundational understanding is necessary.