Lactate measurement: know the limits of technology!

Andrew Hamilton looks at some research on the limitations of lactate measurement during exercise

As we discussed in a recent article (which you can read here), of all the physiological measurements made when testing sport performance, the level of lactate in the blood is without doubt one of the most important. Why is that? Well, the level of blood lactate during exercise shows how efficiently the muscles can utilise fuel and convert it to energy.

In highly trained endurance athletes, the muscles can sustain a very high power output with little accumulation of fatiguing lactate, which basically means that they can work harder for longer, without being forced to slow down by muscle fatigue. In lesser trained mortals, lactate begins to accumulate at much lower power outputs, resulting in a lower sustainable power output. Also, study after study has shown that for any given individual, a lower level of blood lactate for a given sustained power output (for example as a result of an interval training programme) results in better performance. In other words, blood lactate is seen as a reliable performance barometer, with lower levels equating to better endurance performance.

Lactate measurement reliability

Information about blood lactate concentrations undoubtedly gives extremely useful information about performance and changes in fitness levels. But should athletes and coaches rely on the raw data provided by lactate measurements, or are they subject to caveats and interpretation? Most of us assume that with technology comes reliability, and that modern iterations of lactate measurement devices provide highly accurate data that accurately reflects blood lactate levels. The good news is that provided the device is of high quality and properly and regularly calibrated, the lactate concentration figure obtained by the device should accurately reflect the actual blood lactate concentrations measured in the athlete.

While the above might be true however, that’s only part of the story. In many cases of measurements relating to physical parameters, the figure obtained needs to be viewed in context. For example, the figure obtained when measuring blood pressure will depend on a number of factors, including the time of day of the measurement, the foods consumed that day, and the emotional state (eg stressed/relaxed) of the individual. The same is true when using bioelectrical impedence (BIA) machines to measure body composition; the figure obtained is significantly dependent on the hydration status of the subject, and in women, on the time of the month. An obvious question therefore is whether the same caution is required when using lactate measurement devices.

A lesson on lactate measurement from cyclists

An extremely revealing insight into this topic comes from a piece of research on cyclists carried out New Zealand scientists, suggesting that a simple ‘one-off’ test of blood lactate following a session of intense exercise might NOT be sufficient to draw reliable conclusions from – something that could have significant implications for endurance athletes and coaches wishing to monitor performance.

In this study, the researchers set out to investigate how reliable and consistent blood lactate measurements were by taking repeated measurements over time. To do this, 12 healthy, fit and trained male cyclists performed three separate series of power tests on three different occasions. Each series of power testing was identical and consisted of power tests at four exercise intensities: 45% (very light), 60% (light), 75% (moderately hard) and 90% (very hard) of maximum oxygen uptake. The cyclists’ levels of blood lactate were measured pre-test then at the end of every 5th minute, either until the end of the test or until the point at which the cyclists could no longer keep going.

Over the group as a whole, the power outputs at 45%, 60%, 75% and 90% of maximum oxygen uptake averaged 142, 196, 248 and 302 watts respectively across the three testing sessions. What was really interesting however was the variability of the lactate measurements between different testing sessions; although you’d expect to obtain pretty much the same blood lactate measurement for the same power output, what the researchers actually observed was quite a big degree of inter-session variability. Indeed, when they compared the same power outputs in different sessions, they found differences in the measured levels of blood lactate of between 9 and 21%! Interestingly, the higher the exercise intensity, the more consistent the lactate measurement was from one exercise session to the next.

Practical implications for coaches and athletes

Although the research on variability of lactate measurement is limited, the evidence we have suggests that caution is required, and that a single lactate measurement may not be sufficient to draw conclusions from. A coach might be very encouraged to see a drop of 10% in lactate levels at the end of a constant power test following a training programme. But if the session-session variability can be as great as 21%, it would be hard to know how much of the drop was down to an actual improvement in the athlete’s fitness and how much was down to simple variability between testing sessions. This caution would be particularly relevant for tests carried out at more moderate or low intensities – for example to measure lactate concentration at the first lactate threshold.

As the researchers themselves pointed out, coaches should try and factor in the occurrence of testing variability by assessing it as part of their testing. In effect, this means measuring blood lactate concentrations at a given power output on three or four separate occasions to derive an average figure, which would be a much more reliable measure against which to compare. The evidence also suggests that coaches should definitely not rely on a single test of blood lactate at the end of an exercise test as an accurate indicator of change in performance over time as it is likely to be insufficiently accurate.

Reference

  1. J Sports Med Phys Fitness. 2012 Dec;52(6):575-82

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