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Scientific discussion about lactate and lactic acid

Everything you need to know about lactate and lactic acid, the difference between the two, research and theories on why you get “stiff muscles,” and more. Coaching experts bring the topic to discussion.

Photo: Manzoni/NordicFocus

Illustration photo

In this article about training, in collaboration with the private Norwegian team, Team Aker Dæhlie, lactate and lactic acid is the main topic. Written by coaches in Team Aker Dæhlie: Trond Nystad, Knut Nystad, Jostein Vinjeruu, Hans Kristian Stadheim, and Chris Jespersen.

Team Aker Dæhlie was established and launched ahead of the 2022/2023 Winter season, and already in its first season, the private team delivered a track record of performances worthy of a National Team. For the coming season, Team Aker Dæhlie consists of 46 athletes, divided into traditional cross-country skiing and long-distance skiing, women and men, and development teams and para-athletes. 

Also Read: Team Aker Dæhlie for Season 2023/2024

For people interested in understanding the value of blood lactate measurements as a tool to evaluate the intensity of threshold training, one should recognize a need to understand what lactate really is and reflect on why we measure lactate to quantify threshold training. 

Such discussions can often become very technical and theoretical for most people. If you fall into this category, don’t despair because the topic is relatively complex and not as simple and straightforward as many people might think. This is probably because only some who use the terms have fully familiarized themselves with the literature surrounding the terminology or biochemistry but continue to discuss it.

Let’s start with some simple facts. 

Lactic acid is something that is found in fermented foods and drinks, among other things, and gives a “sour” taste. Examples are yogurt and beer. 

Lactic acid was discovered by the Swedish chemist Carl Wilhelm Scheel, who identified the particular chemical as early as 1780. 

Many people, especially in the endurance sports community, use the terms lactic acid and lactate interchangeably. However, the correct distinction is that our muscles produce lactate, not lactic acid, during physical exercise. This happens because of glycolysis (anaerobic energy release), forming lactate and releasing H+ ions (Figure 4). 

Figure 3: Lactate and lactic acid are not the same. Lactate is formed during muscle glycolysis (see Figure 4 further down in the text). Still, it is believed that lactic acid allows you to maintain activity even with high anaerobic energy release in working muscles. 

Sports scientists are still unsure where all the H+ ions come from when the pH drops in working muscle cells (12). To maintain muscle function, the body converts lactate and H+ to lactic acid during anaerobic energy release, resulting in the product “lactate acid” (Figure 3). It is, therefore, somewhat amusing and paradoxical that lactic acid is an acid, while the lactate ion (La-) is the opposite, namely a base. This means that you don’t get “acid” from lactate, as many believe – at least, this is biochemically incorrect.

Blaming lactic acid for “muscle stiffening” during activity was common. The reason for the conclusion was that the amount of lactic acid tended to increase significantly at the same time as the legs “stiffened.” 

This led to the following conclusion: “The lactic acid causes the legs to stiffen.” This is where much of the confusion and conflation between these terms started. 

Firstly, “lactate” and “lactate acid” are similar words that have probably been used interchangeably in English. Molecularly, both substances are also very similar, with the only difference being one hydrogen atom (Figure 3). However, it is the case that neither lactate nor lactic acid causes you to “stiffen.” 

The truth is that we still need to figure out exactly why this happens. 

It has been speculated that this may be due to potassium accumulation in the muscle cells. When an athlete exercises, the cells continuously pump the element potassium in and out. If the intensity is too high, potassium will accumulate outside the cell. This happens because the cell’s “pumps” cannot keep up. The cell doesn’t like this, which can lead to an impaired ability to transmit neurological signals from the brain to the muscle cells. This means that fewer motor units are recruited, and consequently, the force from the muscle decreases, and the pain increases (18). 

In other words, it is difficult to understand exactly what causes “stiffening.” Since it is not possible to measure the potassium content of working muscles during exercise, the practical approach is to measure lactate instead in order to be able to say something sensible about the anaerobic energy contribution. It is, therefore, based on the theory above, important to be a little cautious about concluding the amount of anaerobic contribution based on blood lactate measurements alone.

The introduction of the anaerobic threshold

The anaerobic threshold was first introduced in 1964. In the study by Wasserman and colleagues, they reasoned that “the anaerobic threshold referred to a specific area where a marked increase in blood lactate was observed.” 

They believed this was directly linked to “low oxygen availability” in the working muscles (8). Wasserman’s conclusion is not wrong, but neither is it entirely correct. This is probably due to the previously mentioned “misunderstanding” about the function of lactic acid, lactate, and oxygen.

Figure 4: The end products during aerobic and anaerobic energy metabolism.

We now know that Wasserman’s assumption is incorrect in several cases. For example, it has been shown in dogs working on a thigh muscle (while the dogs were under anesthesia) that there was significant lactate production in the muscle tissue despite oxygen availability being more than sufficient for energy metabolism in the mitochondria (13). 

In later studies, such as the study by Connett et al. (1984), the researchers observed that lactate production was a direct consequence of the rate of glycolysis. This process will always occur with and without available oxygen in the cell (14). 

Another example is that when athletes are “exhausted” after performing hard endurance work, high lactic acid/lactate (over 10 mmol/L) is often measured. However, this does not mean that there is a direct correlation (causality) between fatigue and lactic acid/lactate levels. 

The best example is that during prolonged work (over 90 minutes of continuous work, such as a marathon) where the athletes are exhausted, but the lactic acid/lactate values are rarely very high (i.e., above 4.0 mmol/L). 

Furthermore, it is important to point out that the lactate measured in the blood does not necessarily provide a complete and representative picture of the situation in working muscle cells, as lactate must be excreted from working cells into the bloodstream. 

Among other things, this formed the basis for the “Lactate shuttle theory” introduced by Brooks GA in the 1980s. Here, it was proven that lactate molecules can be transported into the mitochondria but also out of the cell via the bloodstream, from working muscles, and onto other muscle cells. Since lactate retains as much as 95 percent of the energy of the glucose molecule, lactate acts as an efficient energy source. In fact, both the brain and the heart prefer lactate to glucose (12). 

Finally, the document by Nils van der Poel, “How to skate a 10k,” brilliantly summarizes what we believe is a general misunderstanding when measuring lactate in training, not least if you want to use the measurements to say something sensible about whether you are training around anaerobic threshold.

Nils van der Poel “How to skate a 10km” – Short-term blood lactate development:

  • Lower than normal: Indication of a tired body or lack of carbohydrates
  • Higher than normal during training: Indicates good shape
  • Takes longer to stabilize at a low level after a high-intensity set: Indication of a tired body

These remarks from Nils van der Poel are “spot on” and in line with what we see in well-trained athletes. 

One should be careful not to conclude that “high lactate” is always negative and furthermore emphasize that lactate is probably only one of several important pieces of the puzzle in the overall picture to measure and quality assure a good and optimal execution of threshold training in relation to training goals. 

A somewhat ignored and little-discussed physiological response in connection with threshold training is known as “cardiac drift.” This refers to the fact that the heart rate increases by repeating the same speed/workload several times. This increase in heart rate is often explained by a reduction in stroke volume (the heart’s pumping capacity) during the activity.

However, a lot of data indicates that if you experience little or almost no cardiac drift when performing threshold sessions and measure stable lactate values with little variation (+/÷ 0.5 mmol/L difference), you are often within what we define as an anaerobic threshold.

If we then combine these two parameters and include RPE, blood glucose, and see how the athlete “moves,” only then do we get a full-fledged toolbox that can say something reasonable about whether an athlete manages to complete the threshold training with the desired/optimal/planned quality.

The article continues below.

Measuring blood lactate can be important as part of intensity management. But it is important to see lactate measurements in the context of other factors such as heart rate, speed/watt, feeling/perceived exertion, and “how the athlete moves.” Photo: Team Aker Dæhlie

Take home message and summary

To be at your best, you must train with the right personal intensity in as many sessions as possible. Spend time understanding the definitions and experimenting so YOU can find YOUR right intensity. You won’t get better by training at someone else’s optimal intensity. 

Intensity management: 

As a starting point, it may make sense to use Olympiatoppens’s 5-part definitions of intensity zones so you don’t get confused by all the different terms used.  

If you summarize what the best have succeeded with, about 90 percent of the training has been at low intensity (i1/i2), i.e., the most important thing is that you find the right intensity for this training. In total, around 20 percent of sessions, or 10 percent of training time, is spent in i3-i5. Much of this training is carried out as threshold training (i3). 

For most people, threshold training should be carried out at an intensity of around 85 percent of HR max and be a workload that can be sustained for about 60 minutes. It is then helpful to know your threshold heart rate.

As a general rule of thumb, inexperienced athletes over 18 can calculate their threshold heart rate as follows: 211 minus (0.64 x age) = approximately your current maximum heart rate. If you subtract 30 beats from this figure, you are roughly at the threshold heart rate/threshold load. Another method for the more experienced is to do threshold training at about 85 percent of your highest measured heart rate (pulse) over the past year. 

Lactate and lactic acid: 

Many in the endurance sports community use lactate measurement to quantify whether you are actually threshold training. The values one will often state as within threshold (i-zone 3) are between 2.0-3.5 mmol/L, given that the person is in balance. 

Most people should aim to start with values closer to 2.0 mmol/L in the process of finding the right personal intensity. Again, it is important to remember that lactate is only one of several target parameters that indicate whether one is training at threshold. 

For most people, it will be important to view lactate measurements in the context of other factors such as heart rate, speed/watt, feeling/perceived exertion, and “how the athlete moves.” It is not wrong or dangerous to train hard sessions in zones 4 or 5, but it is wrong to plan and document the training as i3 when the execution was in a harder intensity zone. 

To learn the correct composition of your training (percentage in the different intensity zones) and intensity management and achieve good athletic development over time, you must complete as many training sessions as possible at “your” correct intensity and with the proper technique.

Are you interested in traditional and long-distance ski training? Click HERE and read more.


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The complete article can be found HERE

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