What does your “threshold” actually imply?

Over the summer, a physiologist named Karlman Wasserman, formerly UCLA, passed away at the age of 93. The name might not ring, but you can consider your next threshold training as an unofficial tribute to one of the giants in the field. He is the one who was credited with the idea of ​​the “anaerobic threshold” in the 1960s – a concept that initially seemed straightforward but has since proven infinitely controversial and confusing for half a century.

The Journal of Physiology recently published a massive new review of this tangled story, titled “The Anaerobic Threshold: 50+ Years of Controversy,” by four of the most prominent researchers in the field, David Poole, Harry Rossiter and George Brooks and Bruce Gladden. The main takeaway from the paper is that, contrary to the explanations we’ve all heard, the anaerobic threshold isn’t the point where you work out so hard that your muscles can’t get enough oxygen. What, if anything, it represents is the subject of the other 73 pages of the paper. It’s tough sledding, but here are some of the highlights I pulled out of it.

The original idea

The basic concept is that you can divide the exercise into two different zones – call it “easy” and “difficult”. This is not a trivial or obvious statement. Practice could easily exist on a continuum with infinite degrees of difficulty. As early as 1930, researchers spoke of a “critical metabolic level” (known at the time as the Owles point). Below this point, you can exercise without accumulating lactic acid in your blood. above this point you would see a steady rise in lactic acid, which was then believed to cause muscle fatigue. Anyone who has done a lot of endurance training has an intuitive feeling for this transition from sustainable to unsustainable.

Wasserman’s great insight was that you could identify this point by measuring your breathing instead of doing cumbersome blood tests. His assumption was that the critical point was the intensity at which the heart and lungs could no longer supply enough oxygen to the muscles. The resulting lack of oxygen would force muscles to rely on less efficient anaerobic energy sources that produce the dreaded lactic acid as a by-product, and lead to another cascade of chemical reactions that would produce additional carbon dioxide. If you carefully measure how much oxygen a subject inhales and how much carbon dioxide they exhale, a sudden change in the ratio shows that they have exceeded what Wasserman called the “anaerobic threshold”.

The revised idea

Wasserman’s ideas were very influential, but our understanding of what really happens has changed. We don’t actually have any lactic acid in our blood; We have a related molecule called lactate. And lactate is not a dead-end waste product of anaerobic metabolism. It is an extremely useful molecule that acts as an extra source of fuel in muscles and other parts of the body, and a signaling molecule that helps the body adapt and get fitter. And most importantly, lactate is not produced because your muscles cannot get enough oxygen.

The crucial difference between so-called aerobic and anaerobic energy systems is not that one consumes oxygen and the other does not. The point is that one is efficient but relatively slow, while the other is less efficient but can deliver a lot of energy quickly. Once you start pushing hard, you simply cannot provide enough aerobic energy. So you need to start adding anaerobic energy – regardless of how much oxygen your muscles have. This means that hard exercise triggers an increase in lactate production. This is what we would call the lactate threshold these days, and it corresponds to what Wasserman called the anaerobic threshold.

There is another twist. The lactate level you measure in your blood doesn’t just reflect how much lactate your muscles are producing. As mentioned above, lactate is also a valuable fuel, so you use up some of the lactate you make. What you measure in the blood is the difference between lactate production and lactate reuse. One of the big adaptations that come with cardio training is the ability to reuse large amounts of lactate. This means that even if you are consuming anaerobic energy and producing lactate, you can put yourself in a sustainable metabolic state if you are able to use the lactate as quickly as you produce it.

The two thresholds

This new idea – elevated but stable lactate levels – makes the water cloudy because it means there are really two different thresholds. The first is the intensity at which lactate rises above its resting levels; If you exercise just above this threshold, you will have elevated but stable lactate levels. The second is the intensity at which your lactate levels are no longer stable. If you exercise beyond this intensity, your lactate levels will steadily rise until you are exhausted.

Here is a scheme from the Journal of Physiology article showing lactate levels (dashed line) as a function of your exercise (work speed along the horizontal axis). It also shows the two thresholds that divide training into three zones: moderate, difficult, and difficult.

(Figure: Journal of Physiology)

You will find that the labeling of the thresholds is quite confusing. After a count, more than 25 different threshold definitions have been published in the literature using different criteria and nomenclatures. Following the example of the review article, I will refer to the lower threshold as the lactate threshold.

The second threshold is more difficult to determine. It is often referred to as the lactate inflection point (LTP) or maximum lactate steady state (MLSS) as it marks the dividing line between metabolically sustainable and non-sustainable training. However, the review authors note that it is difficult to determine a lactate graph like the one described above because it “represents an attempt to describe a curve with a single data point”. The most reliable way to identify this second threshold is to forget about lactate and use an alternate approach to identify your critical speed (or critical performance which is the same concept in different units).

I’ve written about critical speed a number of times, including this article explaining how to calculate it and how to use it to predict marathon time. In short, if you do three total feats (e.g. races) at different distances and plot them on a graph, you can calculate a theoretical speed that you should be able to run at forever. Of course, you can’t really walk forever – other sources of fatigue intervene. However, this theoretical speed is your critical speed and marks the dividing line between what is metabolically sustainable and unsustainable in terms of the mix of aerobic and anaerobic energy burned.

Take that away

Wasserman’s Anaerobic Threshold was the wrong name (it has nothing to do with oxygen starvation) for the wrong threshold (it’s critical speed, not lactate threshold, that marks the fundamental gap between sustainable and unsustainable). But it produced a lot of good science: the reviewers cite Francis Bacon’s statement that “truth arises from errors rather than from confusion”.

And lactate threshold, whether measured directly from blood stitches or indirectly from the ratio of oxygen and carbon dioxide in the breath, is still a pretty useful concept. Critical speed cannot be measured directly to exhaustion without extensive practice, which is impractical and sometimes impossible. The lactate threshold is more accessible and still has a high predictive value for finding out who is healthy enough to survive major surgery.

In the current context, critical speed seems like the better way to predict marathon performance, as suggested by the recent data dump from Nike’s Breaking2 project. When I reported on Breaking2, the responsible scientific team was also interested in the lactate threshold. The gap between the lactate threshold and critical speed, they told me, gives you additional information about a runner’s strengths and weaknesses. For great marathon runners, the two thresholds are close to each other: they don’t accumulate any lactate until they get very close to their critical speed.

Middle-distance runners, on the other hand, tend to have a large gap between the thresholds: they produce lactate at comparatively low intensities, but it only starts to get out of control at a much higher intensity. Zersenay Tadese, the half marathon world record holder at the time of the Breaking2 attempt, had a high critical speed but a relatively low lactate threshold, like a middle distance runner. That may be one of the reasons he never managed to run a great marathon.

Personally, the threshold concept that I still find most useful is the interview test. It also gives you three practice zones: speaking in full sentences; speak in short sentences; speaking in single words, usually expletively. The University of Wisconsin’s La Crosse physiologist Carl Foster and his colleagues have conducted a number of studies over the years that show how well the talk test accords with more stringent threshold methods. This isn’t surprising: the extra carbon dioxide associated with lactate build-up makes you breathe harder, which affects your ability to speak. As for the details of what’s going on under the hood when you cross a threshold, the physiology may be a little more complicated than previously thought, but one thing hasn’t changed: if you were an endurance athlete, you’d be better able to do it be to feel it.

If you’d like to learn more about Sweat Science, visit me on Twitter and Facebook, subscribe to the email newsletter, and read my book Endure: Mind, Body, and the Strangely Elastic Limits of Human Achievement.

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