By Teemu Virtanen
When we talk about endurance sports, long distance skiing in our case, lactate thresholds and training at those levels are a common topic. Generally speaking, we all know what the term means, but in reality, we may not really comprehend the whole picture. Therefore, it is worth paying a bit more attention to detail in the matter.
Lactate threshold is a widely used term in endurance sports training, and it is one of the most used metrics in the world of training by athletes and coaches worldwide. Lactate formation can occur under aerobic conditions, and that lactate production is the result of glucose utilization by muscle cells under aerobic conditions.
Lactate is the byproduct of glucose utilization by muscle cells. The higher the glucose flux into the cell, the higher the lactate production—independently of oxygen availability. During high intensity exercise, Type II-Fast Twitch muscle fibers are fully recruited, due to high contractile demands by skeletal muscle to produce energy (ATP). Type II muscle fibers are highly glycolytic (they use lots of glucose) which results in the production of high amounts of lactate. This production is a natural by-product of glucose utilization by skeletal muscle cells.
So, lactate is not a waste product since it is the most important gluconeogenic precursor (new glucose generator) in the body. About 30 percent of all glucose we use during exercise is derived from lactate “recycling” to glucose. Lactate is also a key regulator of intermediary metabolism, regulating substrate utilization. It decreases and inhibits the breakdown of fat for energy purposes (lipolysis), as well as the rate of glucose utilization by cells (glucolysis).
During intense exercise, lactate production is many times higher than that of resting levels. The release of hydrogen ions (H+) associated with lactate can cause an important reduction of contractile muscle pH, resulting in acidosis. This excessive accumulation of H+, not only from lactate, but also from ATP breakdown for muscle contraction (ATP hydrolysis), may interfere with muscle contraction at different sites.
Well-trained athletes, such as Pro Team skiers, are very efficient and export less lactate to the blood as they clear it in higher amounts right in the lactate producing muscle which takes seconds or milliseconds. This is very advantageous as it allows contractile muscles a faster H+ removal as well as a faster lactate “recycling” for extra energy (ATP).
During exercise, lactate is mainly produced in fast twitch muscle fibers, which use lots of glucose for energy. It is cleared mainly by slow twitch muscle fibers. This is a complex process involving different lactate-specific transporters and enzymes. Fast twitch fibers have a high content of one transporter called MCT-4 (Monocarboxylate-4) which transports lactate away from these fibers. Slow twitch fibers possess a transporter called MCT-1 which takes lactate inside these fibers. That lactate is then converted to pyruvate in the mitochondria by an enzyme called mLDH (mitochondrial lactate dehydrogenase), to then finally synthesize ATP (energy).
Then, let’s take a look at lactate threshold. It is commonly known as the exercise intensity or blood lactate concentration at the one we can only sustain a high intensity effort for a specific period of time. But it is not simple to determine what that threshold really is as there are questions to be answers such as what is that period of time, what is that blood lactate concentration at, how long can we sustain that given exercise intensity for before we crack down and so forth.
There are multiple theories and hypothesis among the scientific community and not a common consensus of what lactate threshold is. The bottom line to understand what lactate threshold means is that as muscles get more metabolically stressed there is a higher lactate accumulation and H+. Mitochondria in contractile muscles become more stressed to clear lactate in a timely manner and at some point, if the exercise intensity continues, contractile muscle mitochondria become saturated and therefore cannot keep up with lactate clearance, then exporting it to the blood and this is when we see a rise in blood lactate levels which correspond to the metabolic event when it is not possible to maintain that given exercise intensity.
A typical training mistake that many athletes and coaches do is training at lactate threshold in order to improve lactate clearance capacity, but we have to remember that during exercise lactate is mainly produced by glycolytic fibers (fast twitch) which are the ones recruited at lactate threshold. However, lactate is mainly cleared by adjacent slow twitch fibers that have a very high mitochondrial capacity and a much higher amount of mLDH enzymes and MCT-1 transporters. So, it is key to train those slow twitch muscle fibers to stimulate mitochondrial growth and function as well as increase MCT-1 and mLDH.
Training at lactate threshold is essential to improve glycolytic fibers and their machinery and to upregulate the number and function of glycolytic enzymes as well as to increase the number of MCT-4 transporters necessary to transport lactate away from fast twitch fibers to then be cleared by slow twitch fibers. Spending too much time at lactate threshold is very tasking as well, as it is a high effort and can lead to overtraining which is something that happens to a lot of elite and professionals athletes preventing them from reaching their maximal performance potential in races.
