Prior to today, I have completed the lactate threshold guided test on the Fenix 5X three times, and each time I have been unable to obtain a result, “Lactate Threshold not Detected.”

The test consists of a 10-15 minute warm up followed by increasing intensity intervals based on heart rate. Following the warm up, there are four further stages to the test. These consist of 4 minutes in HR zone 2, 4 minutes at the lower end of HR zone 3, 4 minutes at the upper end of HR zone 3, 3 minutes at HR zone 4 and finally 3 minutes at HR zone 5. Once all test steps have been completed, one can only imagine that your lactate threshold pace and heart rate is calculated and fed back to you on screen. As mentioned, each time I have tried this, the display shows a threshold not detected message. Quite frustrating, particularly as it is quite a strenuous workout.

I have come up with a number of theories as to why this has not worked in the past, although cannot confirm that any of them are the true reason for a lack of a result:

  • Poor GPS signal, resulting in a seemingly fluctuating pace, throwing the algorithm off track
  • Shielded running, under trees etc. This was certainly the case for test v3 which was carried out at the War Memorial Park in Coventry, along a path which takes you under heavy tree cover. My assumption is that this could result in the same potential fluctuations as above
  • Running along undulating terrain, causing pace to drop but heart rate to rise, or vice versa, again affecting the algorithm
  • Poor HR management by myself, not being able to remain within the upper and lower limits displayed on the device
  • Insufficient warm up, either by not running for long enough or running it too fast and at a too high heart rate

I should point out that I am not particularly concerned with what my lactate threshold pace or heart rate is. I feel I pace myself well during races and am not actually sure what I could do with it if I knew it. However, a while back now, after giving up after test v2, the F5X somehow managed to calculate my LTHR during an easy run. I’m not sure I went quicker than 9:00 min/mile pace during the four mile, morning run, but once I had finished the watch informed me that my LTHR was 168 bpm and my LT pace was 7:55 min/mile. Given the run that I was on, and the intensity of it, I cannot imagine that this figure is entirely accurate. Again though, I’m not overly concerned. One thing that could potentially pose an issue though, is that my HR training zones were recalculated using this new information.

With that in mind then, along with the fact that I don’t like being beaten, I set out today to eliminate all of the above theories and, hopefully, achieve an accurate result for my lactate threshold.

First of all, I went to the track. Large, open space away from tree and/or building cover and a perfectly flat route/course. Next up, I set the GPS setting on the F5X to GPS + GLONASS. That’ll solve any potential GPS tracking issues and prevent artificial rises in heart rate due to any inclines/declines. Next, sufficient warm up. 10-15 minutes? I ran for 14. Maintaining HR zone 1 was a challenge though, and in those 14 minutes I only managed four laps of the track (1 mile). But I managed it, this was going to go well. The first three 4 minute intervals all went well also, managing to maintain a steady HR throughout, almost bang in the middle of the upper and lower limits stated. I opened my legs for the final two 3 minute intervals, again managing to stay within the stated HR zones. For the final interval, I was all out, running at an average of 6:33 min/mile, which is faster than my 5km pace. It got to the final 5 seconds of the session and I felt confident as my watch bleeped, counting down those final few seconds.

The watch double-bleeped for the end of the test and, you guessed it, “Lactate Threshold not Detected.” I simply do not know what more I can do? Maybe a calibrated footpod would help? Maybe a new HRM? I’m not sure these should really be required. 

Maybe I’ll try again at some point, although I won’t be in any rush to. For now though, I need to decide whether heart rate zones based on the one achieved LTHR result is more accurate than heart rate zones based on percentage of HRmax.

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