The lactate threshold concept

Interestingly, the organism does not wait until the oxygen supply has already become scarce – as one might expect from the term anaerobic threshold. Depending on the level of training, anoxidative glycolysis, the breakdown of glucose into lactic acid, already begins when an untrained person has reached around 30 % of the maximum oxygen consumption capacity and a well-trained person around 50 %. In this situation, there is a measurable increase in lactate in the blood. As lactate is produced without the use of oxygen, the term “aerobic threshold” is often used in the international literature for this first increase in lactate during exercise. However, as less than 5 per mil of the additional energy made available during the first rise in lactate is produced by means of anaerobic glycolysis (see energy metabolism during exercise), I believe that the term _LT1, first lactate threshold, is more appropriate. After all, it leaves room for the important signaling function that the lactate increase has for the organism! During glycolysis, 2 molecules of ATP (chemical energy of the cell) are obtained per molecule of glucose. If you consider that 36 molecules of ATP are formed during the combustion of glucose using oxygen, it is clear that the formation of lactic acid is a very ineffective way of generating energy.

At a normal blood pH value of 7.4, the lactic acid splits almost completely into lactate and hydrogen ions.

As the concentration of lactate increases, the concentration of hydrogen ions in the blood also rises and the pH value falls. However, the body has a huge “arsenal of weapons” that it uses to combat the development of hyperacidity caused by increased lactate formation. The phenomenon that lactate is used oxidatively, i.e. using oxygen to generate energy, plays an important role in less stressed muscles, in the heart, in the brain and also in other organs. As the concentration of lactate in the blood increases, its uptake into these cells is increased, which reduces the rate at which lactate rises with increasing exertion.

In addition to the elimination of lactate from the blood, the various buffer systems play a decisive role in preventing the development of acidosis due to the increase in lactate. By far the most important buffer is bicarbonate, which is formed in the tissue when_CO2 is introduced into water.

The bicarbonate itself can intercept and buffer hydrogen ions, as carbonic acid – in contrast to lactic acid – is only 95% dissociated at a normal pH value. However, the bicarbonate buffer is much more important due to the phenomenon that even a slight increase in the hydrogen ion concentration causes more_CO2 to be exhaled via the lungs (the respiratory system) – an open buffer system! The development of acidosis is largely compensated for by the respiratory system. If the supply of hydrogen ions is triggered by the increased formation as a result of anaerobic glycolysis – i.e. by the metabolism – a “respiratory compensated metabolic acidosis” initially occurs. The lungs thus make the bicarbonate buffer – in contrast to all other buffer systems of the organism – an “open buffer system” with an extremely high capacity.

The amount of lactate measurable in the blood increases significantly with further increases in exercise. An increase in the lactate concentration in the blood of 1.5 mmol/l above the lowest measurable lactate concentration was defined as the 2nd lactate threshold (_LT2). At this lactate concentration at the latest, the hydrogen ion concentration also increases, leading to acidosis with all its negative consequences. In the literature, this threshold is usually referred to as the individual anaerobic threshold, depending on the measurement method, although in this area of the load significantly less than 2% of the additional energy used comes from the anaerobic metabolism!

Many publications assume that from _LT2, from a lactate concentration of 4 mmol/l, the capacity limit for the absorption of lactate in the non-overloaded muscles, heart and other organs is reached, which is why the concentration rises so sharply (MaxLaSS; maximum lactatein the steady state). If this level of stress is exceeded, the lactate will continue to rise even if the stress remains the same, and you will certainly become more acidic. This assumption has not yet been sufficiently substantiated with corresponding experimental data. At higher loads, it certainly takes longer than the 3 – 4 minutes that a load level in the test usually lasts until a safe steady state is reached. Regardless of whether all the assumptions made in the lactate test are correct, many studies have shown that the results obtained can be used to successfully control training.

In summary, two lactate thresholds can be defined, neither of which is characterized by the need for anaerobic energy supply:

At _LT1, a signal is sent to the organism that causes it to undergo various adaptation processes, at least if it is frequently exceeded.

From _LT2, the hydrogen ion concentration increases so much that – if this threshold is exceeded for a longer period of time – disadvantages for the organism are to be expected.

What is the significance of the lactate threshold concept?

The lactate threshold concept presented on this website assumes that the formation of lactate through anaerobic glycolysis is a signal for the body and not the result of a lack of oxygen. The anaerobic provision of additional energy of less than 5 % even at _VO2max is far too low for this.

So let’s listen to our body’s signals – sports scientists have been “eavesdropping” on it for decades.

The lactate threshold concept has clearly proven itself in sports science for controlling training intensity. Today, it can be assumed that a significant training intensity is only achieved with the first increase in lactate, _whenLT1 is exceeded. In addition, many studies suggest that most of the positive effects of sport are mediated via the signal of the lactate increase.

_LT2, for example, indicates that the body is becoming increasingly acidic. A variety of negative effects are to be expected. As an endocrinologist, I must mention at this point that, among other things, cortisol in the blood rises. The catabolic hormone cortisol, which supports the breakdown of protein, favors the supply of amino acids in the working muscle cell for energy production. This results in a breakdown of muscle protein, which is counterproductive when trying to build muscle! The effects of continuous stress above _LT2 on the calcium metabolism are also interesting – although still little studied. Professional cyclists are said to have a lower calcium content in their bones. The picture from the Süddeutsche Zeitung in relation to the coughing endurance runners is only briefly mentioned here.

For the significance of lactate see Chapter Adjusting the training heart rate.