Hey everyone and welcome back to the weekly newsletter.

In last week’s newsletter, I discussed heat acclimatization, the time course for adaptations, and what happens physiologically to enhance performance. In this week’s newsletter, I’ll expound on a post I made last week about a metabolic parameter that might mirror VO_2max. As always, if you are interested in being coached, apply for a free consult at https://www.endurancelaboratory.com/

Over the last few months, I’ve had the privilege of interviewing some of the world’s best middle-distance runners. These include Isaiah Jewett, Isaiah Harris, Cameron Myers, and Hobbs Kessler; each of them has run around 1:44 to 1:43 in the 800m and/or sub 3:30 in the 1500.

While I love talking about their mindset and training, there are two physiological questions I always ask: What is your threshold pace, and what is your 100m dash time?

The most surprising response came from Mr. Isaiah Jewett (1:42 800m PB). His threshold pace was 6:00/mile and his 100m dash time was in the mid to low 10s. In comparison, when I asked Cam Myers (1:44 800m PR) a similar question, he said he ran closer to 4:48 or 5:00 flat for tempo runs, and maybe the mid-low 11s for a 100m dash. How could two runners have vastly different threshold paces, and almost identical 800m PRs?

Metabolism Primer

The answer to this question lies in their metabolic profiles. Before we discuss this, it will help to have a rough understanding of metabolism.

When we discuss metabolism, we generally break it into 2 broad categories: aerobic (using oxygen) and anaerobic (without oxygen). Each of these processes’ primary purpose is to create ATP, the cell’s energy currency in response to increases in energy demand (exercise)!

While I’d love to do a deep dive on the differences in these and why they matter, it’s probably outside the scope of this newsletter. Check out George Brooks’ research [1] if you really want a deep dive.

Within the umbrella of aerobic and anaerobic metabolism, 3 specific energy producing pathways exist. Within the anaerobic umbrella, we have ATP-PCr and Glycolysis. Within the aerobic umbrella, we have Oxidative Phosphorylation. Each of these processes get progressively faster:

·      ATP-PCr is roughly 2–3× faster than glycolysis.

·      Glycolysis is roughly 3–5× faster than oxidative phosphorylation.

·      ATP-PCr is roughly 8–15× faster than oxidative metabolism.

Figure 1 illustrates their relative contribution to total energy consumption during progressively longer exhaustive bouts.

Back to Cam Myers and Isaiah’s lactate thresholds. How could they be so different? Glycolysis always produces pyruvate. When mitochondrial oxidation can't keep pace with glycolysis, pyruvate is rapidly converted to lactate.

In the case of Isaiah, his ability to deliver oxygen to ensure pyruvate is pulled into the mitochondria is much lower than Cam’s. This manifests as a much slower threshold pace. So how can their 800m pace be so different? This answer lies in something called VLa_max, the maximal rate of lactate production, and a proxy of maximal glycolytic rate [2]. Isaiah more than likely has a higher VLa_max and this compensates for his slower threshold pace. In other words, he is a speed based 800m runner.

You might think of VLa_max as glycolysis’ version of aerobic metabolism’s VO2_max [3]. It is a rough measure of maximal glycolytic rate calculated from someone’s peak lactate following a 15 second sprint.

Because lactate is the byproduct of glycolysis when aerobic metabolism can’t keep up, such as in a sprint, the theory is that a runner with higher peak lactates after a maximal 15-second sprint generally has a higher maximal glycolytic rate.

VLa_max explains, in part, why you can’t be an elite marathoner and elite 800m runner at the exact same time. Consider this: runners with a high lactate threshold have faster rates of oxidative metabolism, lower rates of lactate accumulation for any given pace, and faster rates of lactate clearance for any given pace. As such, runners who train and excel in the marathon, will inherently have lower VLa_max because they’ll rely more heavily on aerobic pathways for energy at any given time while runners with a higher VLa_max will have the opposite i.e., higher rates of glycolysis and lower rates of aerobic metabolism.

Here's where the aerobic and anaerobic systems become linked. Lactate isn't simply a waste product. It's rapidly converted back to pyruvate and oxidized by mitochondria. That means an athlete with a large aerobic capacity can clear and use lactate more effectively, while an athlete with a higher glycolytic rate can produce lactate faster than it can be removed.

VLa_max is a helpful parameter because it helps describe whether an athlete is more suited to speed-based training or strength-based training. This is exactly why Isaiah Jewett can run 3 x 400m closing in 47s while Cam might close in 50s. Still very impressive, but vastly different outcomes. As such, I would lean into a sprinter’s strengths if they have a higher VLa_max. I would recommend more speed and speed endurance-based work. If doing VO2_max  work, I would keep the reps shorter with short rest e.g., 10-15 x (30s on and 15s off) at VO2_max.

However, VLa_max is not without flaws. First, its measurement assumes lactate clearance is negligible. It assumes that rate of production far outweighs clearance (probably true). Secondly, it does not mean that someone is incapable of being a marathoner. Isaiah Jewett may turn out to be quite a good marathoner one day if he so chooses. This is because metabolism is flexible, AND VLa_max is agnostic of power. Power drop off is used, but the actual parameter isn’t in the calculation. This is similar to VO_2max. Thirdly, you can almost predict VLa_max from sprint speed [4] if you have two athletes of similar performance levels.

In summary, VLa_max is our best guess at glycolytic rate. Glycolysis is notoriously hard to measure. The most common mistake is to assume that a faster 800m runner has a higher VLa_max in the same way that it’s common for someone to conclude a faster 5k runner has a higher VO_2max. These measures are correlated with power, but power isn’t actually used as a variable in the calculation. If you’ve gotten this far, kudos. This was deep.

References:

1.         Brooks, G.A., The Science and Translation of Lactate Shuttle Theory. Cell Metab, 2018. 27(4): p. 757-785.

2.         Mader, A., Glycolysis and oxidative phosphorylation as a function of cytosolic phosphorylation state and power output of the muscle cell. Eur J Appl Physiol, 2003. 88(4-5): p. 317-38.

3.         Wackerhage, H., et al., Is the vLamax for Glycolysis What the V˙O2 max is for Oxidative Phosphorylation? Sports Med, 2025. 55(8): p. 1853-1866.

4.         Clark, B. and P.W. Macdermid, VLa(max) Correlates Strongly With Glycolytic Performance. Res Q Exerc Sport, 2025. 96(4): p. 660-667.