Reading Research: Anaerobic threshold: the concept and methods of measurement

How anaerobic respiration and lactate/lactic acid actually works has always been one of those things slightly out of my reach.

I have known for a long time that the traditional definitions and explanations have been challenged but I had not really worked through it all properly.  This is my attempt to start addressing that issue.

Sprinters nearing the line – photo by the talented William Warby

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What’s the study?

Well, this is actually a review article and it’s called Anaerobic threshold: The concept and methods of measurement, by Svedahl and MacIntosh, in Canadian Journal of Applied Physiology, 2003.

It’s pretty long and involved but there are only a couple of key points, which I hope I have brought to the fore.  However, please note that there is a lot still to talk about when it comes to lactate and anaerobic respiration and I am only scratching the surface here.

For a bit more on anaerobic respiration and a discussion of the issues surrounding lactate measurement and how it can cause problems in drawing conclusions from the results of studies, check out my article on Energy system contribution in the 200m – 1,500m.

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What is the anaerobic threshold?

The anaerobic threshold is defined by our authors as “an intensity of exercise, involving a large muscle mass, above which measurement of oxygen uptake cannot account for all of the required energy.  Stated in other terms, this is the exercise intensity above which there is a net contribution of energy associated with lactate accumulation.”

That seems fair enough.

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So what’s the problem?

Well, Svedahl and MacIntosh point out that researchers are not agreed on a number of key points when it comes to the anaerobic threshold.  The important points of disagreement surround the following:

• Measuring anaerobic threshold using blood lactate
• The theoretical basis of the concept
• The definitions used

I can’t actually think of anything else they could really disagree about so let’s just assume that this is pretty much a war zone.

As our authors note: ‘there is hardly any important fact concerning the lactic acid formation in muscle which, advanced by one observer, has not been contradicted by some other.’

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Some other framework definitions

To help the discussion, our authors present some (hopefully) non-contentious definitions of the various relevant terms, as follows:

Lactate threshold is defined as “the exercise intensity that is associated with a substantial increase in blood lactate during an incremental exercise test.”

I suspect that this is the threshold that most people who did a little bit of sports science in school or for their personal training qualifications will be familiar with.

Ventilatory threshold is defined as “the exercise intensity at which the increase in ventilation becomes disproportional to the increase in power output or speed of locomotion during an incremental exercise test.”

This is the threshold that we can observe from the outside.  This is where breathing levels out but performance carries on going up.

Maximal lactate steady state is defined as “the highest exercise intensity for which there is an equilibrium between lactate transport into the blood and lactate removal from the blood.”

Now we are getting into the technological side of things.  But fortunately, this one is easy enough to measure.  It’s simply no accumulation of lactate.

Onset of blood lactate accumulation is defined as “the intensity of exercise at which blood lactate concentration reaches 4mM during an incremental exercise test.”

Another technological measure but slightly less discrete.  Simply an arbitrary measure of blood lactate concentration intended to be at a point on the curve at which the accumulation begins to increase.

You don’t need to memorise these to read the rest of the article.  I will repeat them lower down when the time comes to dig into their definitions.

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A brief reminder of cellular respiration

Let’s remind ourselves how energy is formed in a cell.  The important thing to note is that for both aerobic and anaerobic respiration, the first step is the same.  In both types of respiration, glucose is split by a process called glycolysis.  This reaction produces pyruvate, which, in aerobic respiration, is then oxidised in the Krebs cycle.

You can see the two stages below.  The box indicates the specifically aerobic part of the reaction.

So, let’s just think about that glycolysis step a bit more.  During glycolysis NAD+ is reduced, forming NADH.  However, there is only a limited supply of NAD+ available in a cell.  For glycolysis to continue, NADH must be oxidized (have electrons taken away) to regenerate the NAD+ so that the reaction can continue.

In anaerobic respiration, the NADH reduces the pyruvate molecules that are also formed during glycolysis.  Since the NADH has lost electrons, NAD+ regenerates and is again available for glycolysis.  This means that more glycolysis can occur.  Lactate is formed by this reduction of pyruvate.

So lactate accumulation occurs when the production of pyruvic acid and lactic acid via glycolysis exceeds the rate at which these substances are oxidised into the Krebs cycle.

OK so far?

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Measuring anaerobic threshold using blood lactate

So let’s get into the nitty-gritty.  Our authors start their catalogue of difficulties by noting the influential work of George Brooks and other pioneers of lactate research to show that lactate can be oxidized either within the muscle fibre in which it is produced or in an adjacent fibre or another muscle.

Why is this a problem?

Well, if you recall, lactate threshold is defined as “the exercise intensity that is associated with a substantial increase in blood lactate during an incremental exercise test.”

And anaerobic threshold is defined as “an intensity of exercise, involving a large muscle mass, above which measurement of oxygen uptake cannot account for all of the required energy.”

If this is the case, then if we are measuring the blood lactate to determine our lactate threshold or anaerobic threshold, then we are ignoring the lactate accumulation in the muscles in both cases.

As our authors note: “it seems reasonable to assume that if lactate is accumulating in the blood while exercise intensity is constant, then the intensity of exercise exceeds the anaerobic threshold.”

For a bit more on he issues surrounding lactate measurement and how it can cause problems in drawing conclusions from the results of studies, check out my article on Energy system contribution in the 200m – 1,500m.

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The theoretical basis of the anaerobic threshold

Our authors note that misuse of the term “anaerobic” may be an important factor in the pervasive misunderstanding of the circumstances in which lactic acid formation occurs.

They explain that persistent use of the term “anaerobic” has led to the common belief that the presence of lactic acid in muscle is evidence that oxygen delivery was insufficient to satisfy the demand and this is not necessarily the case.

But isn’t this what we all get taught in school?  Why is there a problem here?

Well, our authors explain that it is one thing to note that when oxygen availability is limited, lactic acid will be formed in muscle, but an entirely different thing to then conclude that the presence of lactic acid in muscle means that limited oxygen availability was restricting oxidative metabolism.

The important question is whether lactic acid can be formed in muscle when adequate oxygen seems to be present.  And our authors note a number of studies showing that this is possible.

So what is going on?

Well, our authors note that in Fall in intracellular PO2 at the onset of contractions in Xenopus single skeletal muscle fibers, by Hogan,  Journal of Applied Physiology, 2001, Hogan showed that it is not lack of oxygen that stimulates the glycolysis that results in lactic acid formation at the onset of exercise.

Our authors note: “In Hogan’s study it was observed that oxygen content of single skeletal muscle fibres decreases with a time constant similar to the time constant for the increase in oxygen uptake.  This observation confirms that the relatively slow increase in oxygen uptake at the start of exercise is not due to limitations in oxygen delivery.”

Our authors interpret these results to imply that “oxidative metabolism has a high inertia, and phosphocreatine and glycolysis provide the ATP replenishment while oxidative metabolism is accelerated.”

They conclude that “glycolysis resulting in the formation of lactic acid should be interpreted as a process occurring without the use of oxygen, not necessarily in the absence of oxygen.”

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Using definitions appropriately

Having worked through the preceding two sections, you can hopefully see what is coming here.  Some of the literature uses the definitions of lactate threshold, ventilatory threshold, maximal lactate steady state and anaerobic threshold in ways that are problematic given what we have just read.

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Maximal lactate steady state

Our authors note that if we wish to discuss the intensity of exercise associated with blood lactate remaining at a steady state, then the term “maximal lactate steady state” is a more appropriate manner of referring to that intensity than to say it is the anaerobic threshold.”  Why?

Well, maximal lactate steady state refers to an operational measurement.  You are measuring the blood lactate and recording at what point it ceases to be steady state.  You are not commenting on the implications of that measurement, as you would be if you inferred that it was the anaerobic threshold.

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Lactate threshold

If you recall, lactate threshold is the exercise intensity that is associated with a substantial increase in blood lactate during an incremental exercise test.

Our authors suggest that lactate threshold is probably the term most commonly used in the literature in association with estimates of the anaerobic threshold.  They also perceive that in most cases the use of this term is appropriate.

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Onset of blood lactate accumulation

If you recall, onset of blood lactate accumulation is defined as “the intensity of exercise at which blood lactate reaches 4 mM during an incremental exercise test.”  Our authors note that “this approach to the estimation of anaerobic threshold assumes that the anaerobic threshold is synonymous with an absolute blood lactate concentration of 4 mM.”  This assumption may, of course, not be valid and therefore this metric should not be used interchangeably with the term “anaerobic threshold.”

Our authors also note that the main criticism of this metric is that associating a lactate threshold with a fixed blood lactate concentration ignores individual variability.  On the other hand, the advantage is that it provides a very objective assessment of lactate threshold.

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Ventilatory threshold

Ventilatory threshold is defined as “the exercise intensity at which the increase in ventilation becomes disproportional to the increase in power output or speed of locomotion during an incremental exercise test.”

Our authors note that “the drawback to this approach (and most of these other single test methods) is that it does not necessarily detect the exercise intensity that can be called anaerobic threshold.”

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Conclusions

The key points I have taken away from this article are as follows:

• You can’t measure blood lactate and claim that at the point where it starts to accumulate is the point at which aerobic respiration has ceased to supply all of the body’s energy needs.  Why?  Because lactate accumulates and is oxidised in muscles before it spills over into the blood.
• Because of the accumulation of lactate in the muscles, care should be taken when talking about maximal lactate steady state not to confuse that operational definition with the anaerobic threshold.
• Similarly, care should be taken not to equate the terms “ventilatory threshold” and “onset of blood lactate accumulation” with the definition of anaerobic threshold.
• Glycolysis resulting in the formation of lactic acid should be interpreted as a process occurring without the use of oxygen, not necessarily in the absence of oxygen.
• The relatively slow increase in oxygen uptake at the start of exercise is not due to limitations in oxygen delivery but might be due to the relatively higher inertia of the aerobic pathway.