Accuracy of Lactate Threshold Field Test

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    Most sources online cite the 30 minute field test as the most accurate method by which to estimate your Lactate Threshold Heart Rate (LTHR), the premise being that your average HR over the last 20 mins is the approximation of your LTHR – this is kind of echoed by the 20 minute field test outlined on the 80/20 website. However, a few other sources suggest that, given that the optimal LTHR test is a 60 minute time trial, the average HR in the shorter tests needs to be dialled down, usually by multiplying the result by approx 0.95, yielding a much lower result. Can anyone advise why this isn’t the case in either the 20 or 30 minute field tests please?

    I ask because in a recent 5km TT (23 minutes, so kind of approximates the field test) my average HR over the last 20 mins was 165 bpm, but Training Peaks suggested a revised LTHR of 156 bpm, which kind of fits with the “average HR * 0.95” model. I’m a bit lost on what my LTHR ought to be.

    Many thanks



    The resource I’m referring to regarding multiplying by 0.95 is Joe Friel, in 2020, at:

    A Quick Guide to Setting Zones

    Leyla Porteous

    Hi Pete,
    Apologies if you have already read this resource but here is the 80/20 method for establishing Threshold HR is outlined in this document.

    Intensity Guidelines for 80/20 Running

    Here is an extract that goes through the 80/20 approach.
    “If you have not yet established your TP, you can find your LTHR independently through a time trial. Begin with a warm-up that consists of 15 minutes of easy jogging with a few 15-second surges at the pace you intend to run for the time trial. Next, increase your effort to the highest level you feel you can sustain for 20 minutes and hit the lap button on your heart rate monitor watch. Five minutes into the test, hit your lap button again. Fifteen minutes later, at the end of the 20-minute time-trial, hit the lap button again or end the time trial. Your LTHR is your peak 15-minute average within that 20-minutes test (the average heart rate in beats per minute (BPM) of the final 15 minutes of the 20-minute test).”

    So we are looking at the last 15mins of the test to determine LTHR – this will be the point at which a plateau occurs. You often clearly see this once you go into the file to select the last 15 minutes of the test. We don’t recommend following the Garmin or TP automated threshold change notices as they are not always analyzing the workout as we recommend (ie those last 15mins) a better approach is to go into a session knowing you are going to review the file for any threshold changes and to control the environment as best as possible to ensure any changes are not impacted by external factors (terrain, heat, excitement, nutrition etc. If you are following an 80/20 plan and using the 80/20 zones system then following our recommended testing protocols will ensure everything is aligned and consistent.

    Happy Training

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