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Is lactate threshold a reproducible measurement?

December 31st, 2011

I posted earlier this week about a study that found that the amount of lactate in your blood at threshold doesn’t predict endurance performance. This doesn’t mean that lactate measurements are useless, I pointed out:

It just means that a single lactate measurement in isolation is meaningless: you have to make repeated measurements and track your progress relative to your personal baseline, in order to eliminate the effects of individual variation.

Well, a new study in the British Journal of Sports Medicine actually calls that last statement into question. If you make repeated measurements of lactate threshold, are those measurements repeatable enough to detect small changes in fitness? Many, many studies have examined this question, with the general conclusion that “yes, they’re repeatable.” But most of these studies have only repeated the measurement twice or at most three times, which is hardly sufficient to look for variability.

So researchers from Massey University decided to run a study in which 11 fit subjects did at least six lactate tests each, to see how consistent the results were. The goal here was to make the measurements as identical as possible, so they strictly controlled diet, time of day, and training conditions, all which have been shown to influence lactate values. (Coaches: do you control these factors when you test your athletes?)

Of course, there are many different markers you can look for in lactate tests, so the researchers chose seven of the most common markers: Rest+1, 2.0 mmol/L, 4.0 mmol/L, D-max, nadir, lactate slope index, and visual turnpoint.

These results indicate that only the D-max marker has good reproducibility and that it alone can identify small but meaningful changes in training status with sufficient statistical power.

Expressed in terms of coefficient of variation (with is the standard deviation divided by the mean), the visual turnpoint was by far the worst, with a variation of 51.6%, while D-max has 3.8%. The other markers were between 5.9 and 12.6%. What does this mean in terms of cycling power? They run some numbers to show that even a fitness change corresponding to 70 watts (i.e. improving from 55 to 47 minutes in a 40 km time trial) wouldn’t be reliably detected by most lactate measures.

Tim Noakes, in his accompanying commentary, draws the following conclusions:

[T]hey conclude that unrealistically large changes in power output would have to occur before it can be claimed with certainty that training has produced a real change in an individual’s blood lactate concentrations during exercise. These findings should encourage sober reflection among that large group of exercise scientists who use blood lactate concentrations to guide athletes’ training.

I’m inclined to be a little less negative. After all, coaches and athletes can likely settle for somewhat less rigid definitions of what change can be considered “significant.” As long as they understand that the measurement is fallible and subject to variation, it might still be useful tool for monitoring fitness. (Is it useful for prescribing training paces? That’s a whole different question.)

  1. Marc
    December 31st, 2011 at 19:33 | #1

    Hey Alex,

    I’m only familiar by ‘name’ 2.0 mmol/L, 4.0 mmol/L, the other five markers I assume I’ve encountered in papers but perhaps by different names, but could you translate them for me or just give a brief description/explanation of each.

    Also, if Noakes is so vehemently (with good reason it seems) against using lactate levels to determine fitness improvements and to prescribe training zones, then what system does he use? I would also be interested in hearing his explanation of why many national sports teams (I’m thinking here of the British rowing team) have bothered with utilizing lactate monitoring and HRM and had such success. I’m actually glad to hear these South African swimming coaches have adopted a scientific method (even if it ‘sucks’), he probably wouldn’t be glad if these coaches closed their doors to anything and everything a physiologist has to offer and instead stuck with an out of date method, i.e. pain equals gain.

    Perhaps coaches prescribing self monitored effort zones isn’t so different from what coaches and physiologists are doing when they prescribe specific lactate and HR zones; both systems roughly identify the same intensities for individuals, only the latter at more susceptible to change due to environmental factors, etc. Obviously appealing to something that is measurable, like HR and lactate is more attractive to those of us who want to spend our training time wisely. My only fear is that (and I’ve experienced this first hand) this dismissal of previously accepted methods of monitoring/prescribing effort levels is interpreted by coaches as a license to dismiss the scientific inquiry and understanding of training and proclaim that they have a better ‘understanding’ (I would argue that their real life experience differs from what they read or are told by scientists, but that that doesn’t constitute actual knowledge of what’s really going on) of what’s going on — obviously simply monitoring effort levels via measurable biological products and responses isn’t the same as talking about its actually application to different training methods (how hard, how much, how often), but I think it will eventually lead to more conclusive evidence that show why some methods really are better than others, or how athletes from different teams/countries are able to produce similar results using vastly (perhaps too strong a word) different methods.

    Anyways, hope that mess of a comment make some sense. Happy New Years.

  2. alex
    December 31st, 2011 at 21:17 | #2

    Thanks for the comments and thoughts, Marc. Lots of points I’d like to respond to in there, but I just have time for a quick one right now:

    “I would also be interested in hearing his explanation of why many national sports teams (I’m thinking here of the British rowing team) have bothered with utilizing lactate monitoring and HRM and had such success.”

    Why does almost every daily newspaper print horoscopes?

    Why do so many NBA, NFL and NHL players wear little plastic bracelets with a hologram to optimize their energy flow?

    (I’m not saying that measuring lactate is as stupid as believing in your horoscope. I’m just saying that the correctness of a belief is only loosely correlated with how widely it’s adopted, even by very successful people.)

    Okay, one more point:

    “Obviously appealing to something that is measurable, like HR and lactate is more attractive to those of us who want to spend our training time wisely.”

    Sure, it’s attractive to focus on something that is measurable. That’s the basis for the “Streetlight Effect,” which I blogged about earlier this year:

    “The fundamental error here is summed up in an old joke scientists love to tell. Late at night, a police officer finds a drunk man crawling around on his hands and knees under a streetlight. The drunk man tells the officer he’s looking for his wallet. When the officer asks if he’s sure this is where he dropped the wallet, the man replies that he thinks he more likely dropped it across the street. Then why are you looking over here? the befuddled officer asks. Because the light’s better here, explains the drunk man.”

    Lactate and heart rate are very easy to measure. The light is good there. But that doesn’t mean that’s where your wallet is.

  3. Phil Koop
    January 2nd, 2012 at 17:06 | #3

    I think what Marc is asking is, if lactate or HR measurements cannot reliably indicate how hard you are working to achieve a given power output, what is the alternative? If you do not use these measurements to guide your training intensity, what are you supposed to do instead?

    There is at least one viable answer to this: perceived, subjective intensity. In fact, some training programs offer an alternate calibration based on perceived intensity (e.g. on a 6-point, or 10-point, or 20-point scale) for those who do not possess access to the “scientific” instrumentation. So the issue at stake could be restated as “does the use of lactate threshold outperform perceived intensity as an exercise indicator”. This is a contentious issue; some people think the answer is “no”.

    Looking further into the question, when confronted with the claim that something that is measurable is attractive, my reaction is, attractive to whom? I would say, to coaches and trainers, who have no direct access to an athlete’s interior mental state. Of course, even if there is no objective advantage to objective measurements, there is the possible benefit of placebo effect due to white coats, needles, instruments, etc. It can be comforting to place complete confidence in one’s coach. Sometimes science-ism is about as good as science.

  4. alex
    January 2nd, 2012 at 21:51 | #4

    Thanks for the comment, Phil. I agree with your take, and I think you zero in on a couple of important points. In particular:

    “So the issue at stake could be restated as ‘does the use of lactate threshold outperform perceived intensity as an exercise indicator’.”

    I would actually break this down further into three questions:

    (1) If we had infinitely accurate lactate measurements, would they provide us with useful information to track changes in fitness?

    (2) If we had infinitely accurate lactate measurements, would they offer useful guidance about what pace/effort we should train at?

    (3) If the answer to either (1) or (2) is correct, are real-world lactate measurements precise, accurate and repeatable enough to take advantage of those benefits?

    The study that I blogged about here only addresses question (3), about whether the measurements have high enough quality. According to this particular study, the answer depends on how you interpret the data. The fact that sloppy interpretation of the data can make the measurement useless is something that doesn’t just apply to lactate measurements, of course — it’s pretty much universal!

    Questions (1) and (2) are trickier. My feeling is that (1) is a yes: lactate measurements are a valid way to track fitness. Are they superior to other methods such as, say, looking at your watch and asking how you feel? I don’t know of any evidence that that’s the case — but that’s not the point. As long the method is valid, some people will use it, and more power to them.

    I’m more skeptical about question (2). Is, for example, training exactly at lactate threshold (whatever definition you choose to use) more effective or efficient than training a bit above or a bit below it? I’ve never seen any evidence that this is the case. Heck, the fact that there are so many definitions of threshold argues against the physiological primacy of any one in particular.

    In the end, the lactate curve is, well, a curve — there’s no special place on that curve that’s magically better than the spot next to it. It’s a continuum. And since it’s a continuum, the exact accuracy promised by lactate tests (as opposed to finding your tempo pace based on perceived exertion) doesn’t seem to offer any particular advantage. To me, it seems useful for (a) beginners, who don’t have any sense of appropriate effort levels, and (b) over-driven athletes, who are incapable of not racing workouts. For these people, training based on lactate tests and heart rates might be useful — but it’s not inherently superior to having a coach who simply pulls you off the track and ends your workout if you persist in running faster than the prescribed pace. Either way, you’ve eventually got to learn discipline to race successfully.

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