Let’s start with the concepts of lactic acid and lactate. Those who have studied chemistry know that lactic acid (C3H6O3) is an acid, as the name implies. Lactate (CH3CH(OH)COO−), on the other hand, is the conjugate base of lactic acid. An acid and a base are opposites. There is never lactic acid in the muscles, but rather lactate. Lactic acid and lactate form an acid-base pair, and such pairs are often found together in fluids, with the pH of the fluid determining their ratio.

Anaerobic exercise-induced muscle soreness and burning sensation in muscles.

Another myth is that lactate remains in the body for a long time. This is not true; it is removed from the muscles quickly. However, it is often claimed that lactate (or colloquially, lactic acid) causes muscle soreness after intense training. This is not accurate; the most accepted theory for causing pain is the micro-damage caused by exertion in muscle fascia or fasciae.

During sprinting or competition, athletes often feel a burning sensation in their muscles, which has been explained by the accumulation of lactate in the muscles (or colloquially, lactic acid), but the sensation comes from the accumulation of hydrogen ions in the muscles.

Lactate, on the other hand, is a necessary energy source, not a waste product, and is thus an important part of the breakdown of sugar and energy production. Training increases the body’s ability to improve lactate processing, which raises performance and the threshold above which the body produces more lactate than it can utilize as energy. An athlete can endure this state of stress only for a few minutes before the functionality of muscles or the respiratory system collapses, and the athlete must rest or slow down.

What is lactate?

As mentioned above, lactate is a byproduct of muscle cells’ use of glucose. The higher the glucose transport into the cell, the greater the production of lactate – regardless of oxygen availability. During high-intensity exercise, type II fast twitch muscle fibers are recruited entirely because skeletal muscles contract efficiently to produce energy (ATP). Type II muscle fibers are highly glycolytic (they use a lot of glucose), leading to significant lactate production. This production is a natural byproduct of glucose utilization in skeletal muscle cells.

Lactate is not a waste product, as it is the primary gluconeogenic precursor in the body. About 30% of all glucose used during exercise comes from the “recycling” of lactate to glucose. Lactate is also a central regulator of metabolism, controlling substrate use. It reduces and inhibits the breakdown of fat for energy (lipolysis) as well as the rate of glucose utilization within cells (glycolysis).

Lactate production in muscles

During intense exercise, lactate production is many times greater than at rest. The release of hydrogen ions (H+) associated with lactate can cause a significant drop in muscle pH, leading to acidosis and excessive acidity in the body. This excessive accumulation of H+, not just from lactate but also from ATP breakdown during muscle contraction (ATP hydrolysis), can disrupt muscle contraction at various points.

Well-trained athletes transport less lactate into the bloodstream as a result of intense training. They remove it in larger amounts directly in the lactate-producing muscle, a process that takes seconds or milliseconds. This is highly beneficial because it allows for faster removal of H+ from contracting muscles and faster “recycling” of lactate for additional energy (ATP).

Lactate is primarily produced in fast-twitch muscle fibers during exercise, which use a lot of glucose for energy. Lactate is primarily removed by adjacent slow-twitch fibers. This is a complex process involving various lactate-specific transporters and enzymes. Fast-twitch fibers have a high concentration of a carrier called MCT-4 (monocarboxylate-4 – monocarboxylate transporter isoform), which transports lactate out of these fibers. Slow-twitch fibers have a carrier called MCT-1, which takes up lactate into muscle fibers.

This lactate is then converted to pyruvate (pyruvic acid) in the mitochondria by an enzyme called mLDH (mitochondrial lactate dehydrogenase), which is ultimately synthesized into ATP energy.

What is the lactate threshold?

So, as mentioned above, during intense anaerobic training or competition, the accumulation in the muscles is not lactic acid but lactate, which is needed for energy in maximal performance. Since thresholds were mentioned earlier, let’s move on to explaining their significance.

The lactate threshold is a widely used term in endurance sports training and is one of the most used metrics by athletes and coaches in training. As mentioned in the previous section, lactate formation can occur under aerobic conditions, where lactate production results from glucose utilization in muscle cells under aerobic conditions.

The lactate threshold is commonly understood as the intensity of exercise or the blood lactate concentration at which we can maintain high intensity for only a certain period. It’s not easy to determine what this threshold really is because there are many open questions such as what is the specific period, what is the appropriate blood lactate concentration for each athlete, how long we can maintain a certain exercise intensity, and so on.

Among researchers, there are several theories and hypotheses, and there is still no common consensus on the lactate threshold. The most important thing to understand is that at the lactate threshold, muscles become metabolically stressed, leading to high lactate accumulation and muscle acidity. The mitochondria of contracting muscles become stressed in the process of lactate removal, and at some point, if exercise intensity continues, the mitochondria of contracting muscles become “saturated” or excessively acidic and are therefore unable to remove lactate, causing lactate to enter the bloodstream and lactate levels to rise, resulting in the athlete’s performance decline.

Training at the lactate threshold

A typical training mistake many athletes and coaches make is training at the lactate threshold to improve lactate removal capacity. It must be remembered that during exercise, lactate is primarily produced by glycolytic muscle fibers (fast-twitch) recruited at the lactate threshold. However, lactate is primarily removed by adjacent slow-twitch fibers, which have a very high mitochondrial capacity and a much larger number of mLDH enzymes and MCT-1 carriers. So, it is important to train these slow muscle fibers to stimulate mitochondrial growth and activity and increase the production of MCT-1 and mLDH.

Training at the lactate threshold is, however, essential to improve the function of glycolytic muscle fibers and enhance the number and activity of glycolytic enzymes and increase the number of MCT-4 carriers needed to transport lactate out of fast muscle fibers, which is then removed by slow muscle fibers. Too long training sessions at the lactate threshold may also lead to overtraining, which is common among elite and professional athletes aiming for maximal benefit and performance in their competitions.

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