When running at threshold pace, you are said to be running at an intensity or speed just below the point where your body generates more lactic acid that it can utilise and convert back into energy. You can only run beyond this pace for a short period before fatigue kicks in and you begin to struggle.

Of course, the threshold pace varies from runner to runner (or cyclist to cyclist, other sports are available) and I have been trying to determine where mine lies for some time, although not with any great effort it has to be said. Having carried out a small amount of research online, it appears to be around the pace of a 10 mile / 15 km race. Supposedly quicker than you would run a half marathon, but slower than a 10 km. More precisely, using the VDOT running calculator (found on the site here) and inputting my half marathon time from earlier this year, it suggests that my threshold pace is 10 seconds per mile quicker than the pace that I averaged during the race. Experimenting with other finish times, this also appears to be consistent, there or thereabouts anyway.

If this is the case, then knowing your threshold pace can help you to correctly pace yourself for the duration of a half marathon? Maybe not quite, but knowing your actual threshold pace can help with your training. The best way to lower your threshold pace, or quicken it depending on which way you look at it, is to run at a pace that is slightly either side of it. So incorporating workouts into your plan that involves you running at a pace that is slightly slower and slightly quicker. Therefore resulting in improving your threshold pace and gaining you quicker race times.

There are a number of suggested tests that exist to enable you to discover exactly where your threshold lies. The most accurate of which, is supposedly the 30 minute maximal test. This workout involves a sufficient warm up, followed by 30 minutes of running at a pace that you can sustain for the 30 minutes, but not for any longer. Your average pace for the 30 minute duration is your resulting threshold pace. I’m not entirely sure how this relates, given that it is said to be your 10 mile pace, which you would be able to sustain for considerably longer than 30 minutes, but this is what is recommended.

The Fenix 5X has a workout feature on it called the lactate threshold test. Following a warm up, the watch guides you through 3-4 minute intervals of increasing intensity, starting with 4 minutes in zone 2 and finishing with 4 minutes in zone 5. At the end of the test, in theory at least, the watch calculates and provides you with your threshold pace and heart rate. I have now carried out this test twice and both times have received a result of “no threshold detected.” Although it is a good workout, working that hard for around 20 minutes to be given a non-result is very frustrating.

Imagine my surprise then when, following an early morning easy run where I only entered zone 4 twice whilst going up hills, the F5X stated that lactate threshold had been detected. 168 bpm and 7:55 min/mile. The watch also asked me if I would like to update my HR zones based on this new reading, to which I confirmed yes. Subsequently, my zone 2 and 3 easy and aerobic has been replaced with low aerobic and high aerobic, with the 168 bpm now being the limit of zone 4 before zone 5 begins.

Exactly how accurate this is I am not too sure, however it is slightly quicker than my pace for the half marathon that I completed in March, which would suggest a reasonable level of accuracy, although I would have expected it to have improved since then. Either way, this is a reading that I can now work with, even if it is around 20 seconds per mile slower than my target pace for September’s Northampton half. Not worrying too much about that for now, at least I now have a target for my threshold runs.

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