Calculating Zones (20min TT)

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    Today I performed a 20min time trial in order to estimate my Threshold Pace (TP) and LTHR. I started with a 15min warmup including a couple surges as prescribed in the Intensity Guidelines, and did the 20min at (close to) maximal effort. Analysing my data in TrainingPeaks, my average pace for the entire test was 4:21 min/km, and my average HR for the last 15mins of the test was 176 (trimming the initial 5mins to remove effect of cardiac lag).

    The pace zones calculated by using the “20-minute Test” function in the 80/20 calculator by inputting 4:21 seem reasonable (from past experience). However, to estimate my LTHR and be able to calculate my HR zones, would I not take 95% of the 176 value (0.95*176=167.2) and call this the LTHR? I’ve read elsewhere that the more accurate way to estimate LTHR is through a 60min TT, but for practical purposes many athletes opt for the 20min version instead, taking 95% of their avg HR during the 20min TT since that is approximately the intensity they would sustain for the full 60min effort.

    From past experience, this 176 value seems way too high to be my LTHR, seeing as the highest I’ve seen my HR ever go is 186. How would you calculate your LTHR from performing a 20min TT?

    Anyone with some insight, please let me know! Thanks.


    Following. I just took the last 15 mins also.


    Hi Nik,
    The last 15 mins of the test where your HR levels off is your LTHR, you don’t take any further calculations to determine HR as you would with power in a cycling FTP test. So whatever the peak HR average is in those last 15mins that is your LTHR info for that test. IT’s important to note that heart rate is significantly influenced by factors such as temperature, humidity, sleep, stress, time of day, and even when you last ate. Therefore, your Lactate Threshold Heart Rate test result is specific to the environment and conditions in which you test. For example, an LTHR test indoors in February in the morning will not be the same as an LTHR test in July outdoors in the afternoon. Perform your LTHR in the environment that most accurately represents where you will do the bulk of your training.

    For various reasons, most athletes find that their threshold is 5-10 bpm lower indoors than outdoors. For this reason, it may be practical to maintain separate indoor and outdoor HR zones. Or simply adjust your zones appropriately when moving between outdoor and indoor environments.

    This may explain why you are seeing a various to previous LTHR data – and why it’s important to all the factors that impact HR into using and interpreting LTHR info in your training.

    Hope that’s helpful

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