Lactate often gets demonized for limiting performance and causing that painful burn in active muscle. Coaches will schedule recovery workouts 24 hours after competitions to “flush the lactic acid out of the legs”. However, a more in depth understanding of lactate would show you that lactate isn’t as evil as some may perceive it to be and is often quite misunderstood.
At moderate intensities of exercise, lactate does not accumulate significantly in the blood because aerobic metabolism facilitates lactate clearance at an equal level of production. However, when aerobic metabolism cannot keep up with energy demands, lactate quickly accumulates. This is known as the lactate threshold, the level of intensity where lactate quickly increases above (roughly) 2 mmol/L. In the figure below we can see that as exercise intensity increased on the cycle ergometer to just under 200 watts, blood lactate quickly started accumulating.
When the body needs high amounts of energy quickly, ATP must be derived from anaerobic processes because these systems can produce ATP fast, but not for very long. The phosphagen system can support ATP production via substrate level phosphorylation for up to about 10 seconds, give or take. The metabolic by-products of this (inorganic phosphate and ADP) stimulate our next energy system, glycolysis. Glycolysis is the process of splitting glucose to derive ATP and is facilitated by its rate limiting enzyme, phosphofructokinase. The glucose often comes from glycogen stores within the muscle. The metabolic by-products of this process are what ultimately lead to lactate formation. It is the accumulation of hydrogen ions that become problematic. At lower intensities the hydrogen ions are used in the krebs cycle via pyruvate to generate more ATP, eventually making their way to the electron transport chain (an aerobic process). However, at high intensity levels, pyruvate converts to lactic acid and then lactate.
In the short term, lactate is helpful in enabling glycolysis to continue as hydrogen ions left alone are destructive and immediately effect blood acid levels. Lactate serves to briefly delay this process.
What many coaches fail to realize is that lactate serves as energy substrate as it can be converted back to glucose via gluconeogenesis in the liver via the Cori cycle. Additionally, inactive muscle tissue can metabolize lactate to derive ATP. This is how active recovery between high intensity bouts can facilitate recovery. Therefore, lactate isn’t bad per se, it is necessary.
Lactate levels can accumulate quite high in the blood, but generally return to baseline levels shortly after exercise cessation (generaly < 60 minutes). The concept of performing low intensity exercise a day after the competition to facilitate lactate clearance is quite silly. That’s not to say that other benefits cannot be derived from this, but lactate clearance isn’t one of them. Overall, lactate isn’t quite as bad as most people would lead you to believe.
