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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.)