In a rush to make it to the Tuesday night club training session on time, I left the house without my chest strap heart rate monitor. This meant relying on my watch to determine my heart rate during the session. I must admit, during hard sessions at least, I don’t tend to review my heart rate data during the run, but I do like to look back at it afterwards to gauge the relationship between perceived effort and heart rate.

Tuesday night’s session was a tough one. 2 x 5:30, plus 3 x 2:40, with 3 minutes rest after each effort. It’s fair to say that I was pushing it hard, clocking some of the quickest splits that I have ever achieved. So it was a surprise when I switched my watch to the heart rate screen midway through the second 5:30 effort, to see that my heart rate was 136, easy apparently. It was certainly not easy!

I didn’t check it again until later that night. The maximum HR that the watch recorded was 164, with an average of 142. I wasn’t too surprised by the low average as I left the watch ticking over during the rest periods. I was surprised however by the maximum, as I would have expected to get close, or even reach, my maximum heart rate during such a hard session. Giving it everything I had during the last 2:40 produced a HR reading of barely 150. Unfortunately I don’t have any supporting data to compare it to, but that seems considerably off to me.

This mismatch/incorrect/inaccurate reading, however you wish to look at it, would not usually cause any issues, aside from a bit of frustration maybe. However, given the features on the new Fenix 5X, this does actually have some repercussions. Using Firstbeat, the F5X provides you with an aerobic and anaerobic rating for each activity that you do. Using this reading, the watch then calculates your exercise load for the last 7 days, whilst also providing you with a V02 Max estimation. For Tuesday’s particular run, I received a 2.6 aerobic rating (out of 5) and a 1.0 anaerobic rating. According to the watch, and therefore Firstbeat, this equates to “maintaining aerobic fitness” with a “minor anaerobic benefit.” Presuming the HR data was incorrect, this information, and subsequent V02 max estimation, race finish time predictions and weekly load reading are all useless, and ultimately misleading. Disappointing.

I should point out that, during normal wear, I have found the wrist heart rate measurement to be very accurate. When idle, this can be cross checked by feeling my pulse and counting the beats. And by accurate, I mean within 2-5 beats per minute. It’s just during activity that it seems to be out. During the recent East Midlands Grand Prix series, I didn’t wear a chest strap and I found the HR readings to be equally out, with maximum readings of around the 160 mark. I would expect considerably higher than that during racing, particularly at shorter distances.

Unfortunately I am also unconvinced by the readings shown recently by my chest strap. On Friday I went out for my weekly endurance run and, for a large part of it, I was running at what my heart rate would suggest as threshold. Endurance runs should be at an easy pace (which it was) that is conversational (which it was) but my chest strap, via my watch, was telling me that 9:00 min/mile is my threshold pace, and I wasn’t even travelling uphill. Ideally the easy run of the week should be performed in HR zone 2, but no matter how slow I go, I just cannot keep my HR below 140.

Maybe I am more unfit than I thought. Maybe my watch is actually correct, I am fitter than I thought and I should be running and racing harder than I am. Or maybe both the watch and chest strap are out and I need to invest in a new chest strap. You would certainly usually expect a chest based monitor to be more accurate than a wrist one.

With hindsight, I should have done a comparison test before passing on my old watch, so I could see the live differences between the chest strap and the wrist monitor.

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