The BLUE curve shows the lactate combustion rate. To recover from hard interval efforts, we use an active recovery period in between to keep metabolism running, because lactate gets combusted faster when we move. Or, more scientifically: the only way to get ”rid” of lactate is to use it as a fuel in the aerobic metabolism, and that is why you should picture lactate as a “fuel for the muscles”. So, once we acknowledge that the primary pathway to use lactate is to use it as a fuel in the aerobic metabolism, it should become clear that lactate combustion rate in the aerobic metabolism is given by the rate of the aerobic metabolism itself.
Or in other words: the higher the aerobic rate in a given metabolism, the more lactate can actually be used as a fuel. And this is what the blue curve shows, i.e., the maximum aerobic combustion rate of an athlete. This rate origins from the oxygen uptake rate displayed in the graph above, and it is expressed as a flux rate: mmol of lactate per minute. You may also look at it as the gross lactate combustion rate in the aerobic metabolism.
The RED curve shows the actual lactate production rate. The lactate production rate depends on the anaerobic – or, more precisely, on the glycolytic power of the athlete. The higher this rate is, the more lactate he or she will produce at any given intensity.
One of the most interesting state of energy metabolism is the crossing point of lactate production and lactate combustion (red and blue curve). One of the most interesting state of energy metabolism is the crossing point of lactate production and lactate combustion (red and blue curve). At this intensity, both pathways (the aerobic and the glycolytic one) run at the same rate, and the lactate production equals the lactate combustion rate. This point marks the highest possible intensity at which lactate concentration would stabilize at a certain level (steady-state). At an intensity below the crossing point (left-hand side), the maximum possible combustion is always higher than the actual production. At an intensity above the crossing point (right-hand side of the graph), the production rate exceeds the combustion rate, lactate accumulates more and more, and there is no possible steady state of lactate concentration. That is why the crossing point marks the maximum lactate steady-state or anaerobic threshold.
The crossing point has been validated numerous times by comparing the power/speed at the crossing point against the gold standard: 30-minute tests at a steady power and monitoring the lactate concentration to see if a steady state can be reached. Those verification studies have been performed in several sports with males and females (something usually not possible with simple lactate profile tests). The average tolerance of the power/speed at the crossing point compared to the gold standard is always in the range of 2 – 2.5%. This is significantly lower than any other physiological test aiming to determine the anaerobic threshold.
The YELLOW curve shows the lactate concentration in the blood at steady-state conditions. What you may find is that lactate concentration is somewhat lower than expected at a lower intensity and, unexpectedly, higher at higher intensities. This is because it is in steady-state conditions, meaning that time is infinitive long. For a low-intensity effort, you might be well aware that lactate concentrations decrease over time (compared to what you would see at the same power/speed during an incremental lactate profile testing protocol), to levels even well below resting level. This is what you see in the yellow curve. Now at high intensities, above the threshold, there is no steady state of lactate concentration anymore as it accumulates over time. And therefore you can see the steep increase in the yellow curve.