Lactate, or lactic acid, is a by-product measured by sport physiologists and coaches to evaluate workouts and profile athletes during exercise testing. Lactate is not the cause of fatigue, but does correlate with the exercise intensity, thus making it a good biomarker for performance.
Lactate is the end product of glycolysis. Glycolysis is the process of using carbohydrates as a fuel in the glycolytic (anaerobic) energy system to produce energy.
Athletes start to use more carbohydrates as a fuel with increasing exercise intensity and therefore, more lactate is produced. This made people think that lactate is a
by-product that causes fatigue because at high exercise intensities, lactate concentrations are high, and athletes fatigue. However, there is no causal relationship between lactate and fatigue. The produced lactate can even be used as a fuel: the aerobic energy system uses this lactate as a fuel to produce more energy.
At a certain point, the lactate production exceeds the lactate elimination (lactate threshold) and this will cause an exponential rise in lactate values. Above this point, the athlete can no longer sustain his effort due to the accumulation of lactate. As a result, lactate can be used as a good marker for monitoring fatigue.
Lactate values give you more information than speed or power (external load) and local muscle activity (internal load) because it is a global indicator of intensity from work and tells you what is happening inside the body. Heart rate is another useful indicator of exercise intensity, but its reaction is slow and it only indicates the reaction of the heart on what is happening inside the body during exercise.
Lactate values are adding a layer of precision and personalization to training which give useful insights to the coach and athlete.