I hit my LTHR (Lactate Threshold Heart Rate) during a 5k PB at around the 1.5k mark.
So what? You rightly ask.
Well the LTHR is a point that when you perform above that point your body starts to accumulate too much lactic acid/lactate/H+ ions which, at SOME POINT, will cause you to stop.
But where is that ‘SOME POINT’. Well clearly for me based on that race it is AT LEAST 3.5kms (ie from 1.5km to 5km) and in fact I have stayed above it for 26 mins in a 10k and for 37 mins in a duathlon. In fact if you are fitter than me then you can keep at that level for over an hour…but probably not for a Half Marathon or a 2 hour duathlon/triathlon.
Going back to 5km then people can, apparently get their HR above that level for all but the first 30 secs or so. I’m not sure how though. As in my case to get my HR that high would require me to run a VERY fast first KM – which kind of goes against the idea of an equal split over all the 5ks of a 5k race to get a PB.
Any thoughts from your experience?
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Last Updated on 13 December 2025 by the5krunner
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tfk is the founder and author of the5krunner, an independent endurance sports technology publication. With 20 years of hands-on testing of GPS watches and wearables, and competing in triathlons at an international age-group level, tfk provides in-depth expert analysis of fitness technology for serious athletes and endurance sport competitors.
