In the previous blog we talked about the importance of knowing fundamental running metrics like VO2max, VLamax, Fat combustion and Carbohydrate combustion. We saw how knowing those metrics makes you a faster runner instantaneously. In this blog we’ll show you how knowing your lactate accumulation and recovery will optimize interval training.

How VO2max and VLamax interact

The Aerobic system burns carbohydrates and fat to produce energy. The Glycolytic system prepares the carbohydrates by using glucose and producing lactate to fuel the Aerobic system. The Glycolytic system produces energy in doing so. There are three states at which those two energy systems interact:


  • 1. When intensity is low, the Glycolytic energy system does not produce a lot of energy. This also means, that it does not feed the Aerobic system with a lot of fuel (lactate). Since the Aerobic system could use some fuel, we call this low intensity situation: lack of lactate or lack of pyruvate. To compensate for this lack of fuel, it needs to burn another fuel: fat.
  • 2. When intensity increases, more and more energy will come from the Glycolytic energy system. This automatically means, that it will use more glucose and produce more lactate to fuel the Aerobic system. When the Glycolytic system produces as much fuel (lactate) for the Aerobic system as the Aerobic system needs, there is no lack of lactate anymore. You might have heard that some call this the Anaerobic Threshold or FTP. Since there is no lack of lactate, it is not necessary to burn fat anymore.
  • 3. When intensity increases even further, we come into a situation in which the Glycolytic energy system produces more fuel (lactate) for the Aerobic energy system than it needs. Lactate will accumulate. Although this accumulation of fuel (lactate) will not cause fatigue, it is a good marker for fatigue.


Image 1 represents the above-mentioned situations 1, 2 and 3. At 5.1 m/s (x axis) lactate production by the Glycolytic system equals lactate combustion by the Aerobic system. At intensities below this so-called Anaerobic Threshold or FTP, there is a lack of lactate (also called: lack of pyruvate). At intensities above 5.1 m/s, lactate will accumulate.

Image 1. At intensities below the Anaerobic Threshold (1), there is not enough lactate (lack of pyruvate) to fuel the Aerobic system. At intensities above the Anaerobic Threshold (3), lactate accumulates.

How to use lactate accumulation and -recovery to optimize your interval training

Let’s look at two examples that will immediately clarify how you can use the previous information to optimize your interval training!

Example 1: recovery time

Imagine you want to train at a race intensity that you cannot sustain yet. Take for example the athlete of image 1. If this athlete runs at 5.5 m/s (x axis), lactate accumulates 1 mmol/l/min (y axis). After say 3 minutes, the lactate accumulation is 3 min * 1 mmol/l/min = 3 mmol/l. If the athlete wants to recover, he/she needs to take into account that:


  • To combust the lactate that accumulated, we need to run at a speed/pace below Anaerobic Threshold. Makes sense, right?
  • There is an optimal intensity to combust lactate, which does not appear at the very lowest intensity. Intuitively, this also makes sense. During recovery we often keep moving. It would not feel good to lay down on the road and start a new interval immediately afterwards.


Image 1 shows that our example athlete recovers fastest from lactate accumulation at a running speed of 3.6 m/s. He/she then combusts 0.9 mmol/l lactate per minute. It will take 3 mmol/l ÷ 0.9 mmol/l/min = 3 minutes and 20 seconds before the athlete combusted all the lactate that accumulated. If he/she maintains this running speed, fat will start to deliver the lack of fuel again. Since we are recovered, we could also choose to add another interval.

Example 2: interval work-to-recovery ratio

The graph also teaches us about the work-to-recovery ratio of an interval training. Obviously, the higher the intensity during an interval, the more recovery we need. The graph easily uncovers the ratio and intensities. Image 2 shows how long we should recover from any intensity. Every minute we run at ~5.5 m/s needs one minute of recovery (at ~3.6 m/s). Every minute at ~5.8 m/s needs 2 minutes of recovery, etc.

Image 2. Want to know more about work-to-recovery ratios? No problem! Pick any intensity above Anaerobic Threshold and see how long it takes to recover.

The previous two examples both represent a perfect balance between lactate accumulation and lactate combustion. In other words: lactate levels before an interval are the same as after the recovery. You could of course also choose to train at an elevated lactate level throughout the whole set of exercises. For instance, by doing a hard 2-minute effort before you start your 1 minute on/off intervals. This is only possible because you can now perfectly balance your intervals, and therefore prevent too much or too little lactate accumulation (again: a marker for fatigue).

In summary

We again see the importance of knowing the two fundamental metrics: VO2max and VLamax. As a result, you get to know your lactate accumulation and recovery, which are markers for fatigue. Using those metrics enables you to learn at which pace you recover fastest and helps you create the perfect interval workout. The next running blog is about indoor vs outdoor testing for runners: which one is better?