Is VO2max correlated with running performance?

Artistic scatterplot showing 3k race times as a function of VO2max

Quite often I hear people claiming that “VO2max is not correlated with running performance.” Is that true?

What about similar correlation-based claims about metrics like body composition, running economy, mileage, and maximum heart rate?

Usually, when people make these kinds of claims, they link to a study showing that, among a certain group of runners, the metric in question (e.g. VO2max) does not accurately distinguish between the fastest and slowest runners in the group.

Another version of these claims comes in the form of statements like “among runners with a similar VO2max, those with better (running economy / lactate threshold / some other performance metric) have faster race times.”

So, what should we make of these kinds of claims? My goal in this article is to show why correlations will always become weaker when you restrict your analysis to a small subset of a population, like elite athletes.

Let's dig in and see why.

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Iron supplements, ferritin levels, and VO2max gains in athletes

Red blood cells in the style of a hand-drawn sketch

There’s a new study out this month on iron supplements for athletes with low ferritin. The study, published by Anja Neža Šmid and colleagues at the University of Ljubljana in Slovenia, is a meta-analysis, meaning it pooled data from several different randomized trials that took athletes, assigned them to either a placebo group or an iron supplement group, then measured the difference in ferritin levels between groups after the study’s conclusion.

The results of this meta-analysis are some of the strongest evidence that I’ve seen that support the notion that athletes with ferritin levels circa 20 ng/mL will benefit from an iron supplement. However, I wanted to write up this post because that’s actually not how the authors of the study interpret their own results!

Here’s what Smid et al. say in the abstract:

Increase in serum ferritin concentration after [oral iron supplementation] was evident in subjects with initial pre-supplementation serum ferritin concentration ≤12 µg/l [ ng/mL], while only minimal, if any effect, was observed in subjects with higher pre-supplementation serum ferritin concentration.

Šmid et al 2024

How can I justify the benefits of iron supplementation at 20 ng/mL, while the authors cite a cut-off value of barely half that level? Well, it all comes down to the way you analyze the data.

Here’s the bottom line up front: when treating ferritin level as a continuous number—which it is—the study’s data clearly show that runners with ferritin levels of around 20 ng/mL or less will experience significant increases in ferritin and gains in VO2max when taking an iron supplement over the course of 6–8 weeks.

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Problems with the critical speed model: Can power laws predict running performance better?

Digital art image of a runner with math equations in the background

The critical speed model—also known as critical velocity, CV, or critical power—is a powerful concept for understanding what running speeds are sustainable at a metabolic steady-state and what speeds are not.

Critical speed is not without its detractors, though, and the critical speed model is certainly not without its flaws.

I just posted a huge article on understanding the science of critical speed, critical velocity, and critical power for runners. That article goes in-depth on what critical speed is, how the model works, and how you can use it in your training.

I was originally planning on including the major criticisms of critical speed as a part of that article, but it’s long enough as it is.

So here, separately, is a summary and analysis of the main problems with critical speed (and, by extension, critical power) as a model for endurance performance, plus some rejoinders as to why alternative models, like the power law model of performance, are not always better.

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The science of critical speed, critical velocity (CV), and critical power training for runners

Female athlete running in front of scientific graphic paper

Critical speed is the boundary that separates running speeds that can be sustained at a metabolic steady-state from speeds that cannot. Sometimes called critical velocity or “CV,” critical speed is known in the running world in partly due to its popularization by Tom “Tinman” Schwartz and his proteges, including Drew Hunter.[1]

Critical speed is increasingly becoming the gold standard among physiologists for identifying the limit of what runners would call “high-end aerobic” or “steady-state” running speeds, and is gaining traction as a training tool as well. The critical speed model explains the body’s response to different speeds better than older models based on the lactate threshold.

Among exercise physiologists, critical speed (or a semi-related concept, the maximum lactate steady state, which we’ll also discuss) is rapidly becoming the gold standard for capturing the aerobic fitness of athletes.

Critical speed has its roots in early work in the 1960s, 70s, and 80s, but didn’t really start to emerge as the strongest physiological model for intense exercise until the last 15 years or so.

In this article, we’ll take a detailed look at the critical speed phenomenon, understand how it works on a mathematical and physiological level, see some of the problems and controversies surrounding it, and learn how to apply the concept of critical speed in your own training.

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A comprehensive overview of Canova-style percentage-based training for runners

Percentage-based training is a mathematical approach to planning workouts for runners. Percentage-based methods are used by many top international coaches, most notably Renato Canova, to train runners at distances from 800m to the marathon. 

I could write a whole book about percentage-based training for runners (in fact, maybe I will someday), but the goal of this post is to give a clear, comprehensive, and readable overview of percentage-based training as a system—a set of principles that can be used to guide training decisions.

To this end, we’re going to focus on the concepts and rationale behind the percentage-based training method, as opposed to exact training calendars. This post does include an appendix with recommended workouts for every event from 800m to the marathon (even the 3k!), but event-specific full training calendars will be a project for another day.

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How much glycogen is stored in a runner’s liver?

Carbohydrate-rich foods in the shape of a liver in the style of a scientific illustration

Liver glycogen is a major source of carbohydrates for your muscles to burn when you’re running. The glycogen molecule is just a big long chain of glucose molecules which is optimized for long-term storage.

When your body needs carbs during a run during a run, your liver can break down glycogen into glucose, shuttle it into the bloodstream, and send it to your active muscles to be burned for energy.

Stored glycogen in your liver is particularly important for running the marathon and ultra distance races, since the glycogen that's locally in your muscles isn’t sufficient to get you to the finish line. But exactly how much glycogen do you store in your liver? And can training increase this amount?

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Introducing Running Writings Apps and the workout pace percentage calculator

Screenshot of the Running Writings workout pace percentage calculator web app

This week I launched Running Writings Apps, a new platform for me to host some of my more technology-heavy running-related projects. During my PhD work I did a lot of data analysis and data visualization, and picked up some strong programming skills along the way. I’m excited to incorporate more of that alongside my long-form writing here.

This week I launched my first project: the Running Writings workout pace percentage calculator! Check it out!

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Training intensity and capillary growth: Do you believe all the studies, or just a few? 

Color microscope image of muscle fibers and capillaries

Recently I’ve been diving back into the world of exercise physiology research, especially as it relates to running performance. I got behind on following ex phys in grad school (too busy following biomechanics!), so it’s been a few years since I’ve caught up on the latest research.

This weekend I was reading about capillarization: the growth of new, tiny blood vessels–capillaries–that run between and around individual muscle fibers. Capillaries are super important for aerobic performance, since they’re the place where oxygen diffuses out of red blood cells and into muscle fibers. 

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The Keys to Marathon Training: Modern changes to Renato Canova’s elite marathon training methods

While researching my blog post on Renato Canova’s marathon training book, I came across a lecture that Canova gave at a coaching conference put on by Spanish marathoner and coach Antonio Serrano in 2017. The talk, called The Keys to Marathon Training[1] was held in conjunction with the 2017 Valencia Marathon.

This lecture directly answers one of the questions I had when writing up my analysis of Canova’s book–what’s changed since 1999? From his answers in a 2011 interview, I knew that Canova believed some important things had changed, but that video didn’t go into too much detail. Canova’s talk at this conference goes into much more depth, so I wanted to do a more formal write-up on it.

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Did you know I have a book? Check it out here!