Heart rate monitors, how functional are wrist based monitors?

So some background on my use with the HR built into my fenix 5x, training for a marathon I foound a lot of really weird anomalies using it.

For example, running my dogs at a Z2 pace, I would see averages in the 160-170bpm. Then I could go do a tempo run and see averages around 120bpm. Neither result where I expected. Yes, I tried every possible way of wearing the watch to get the results I should expect. Some days it seemed correct, more often then not, it was useless.

I moved to a chest strap for training last year and felt it was always correct with what the level of effort was. Great, except I don’t always wear it.

Which leads me to yesterday… mountain bike ride, Z2 effort. I mount my watch on my handle bar now with the Garmin bar mount. No chest strap. Start riding and the watch is reading a pulse🤔. It’s reading around 90-120… not far from where I would expect it, except I had nothing on me to detect it with.

Is the wrist based system “guessing” at my HR? How much can one take from HR data if the data is made up?

If you still have the sensor on then the watch is still trying to determine heart rate, it doesn’t know if it is on your wrist or not.

You can turn the sensor off, like you I found the Fenix HR to be inaccurate so I stopped using it and turned it off to save battery power, though I don’t know how much it uses.

My Fenix 5 seemed to do HR pretty well, until I started doing something and then it wasn’t very useful.

1 Like

I have a Fenix HR and the optical sensor is garbage. Use a chest strap.


I think I’ll just turn it off as you mention.

But the thing is, if it can really measure heart rate, it shouldn’t matter if it’s on my wrist or not. If it’s not on my wrist, it should be reading 0 or - -. The measurement while it’s not on my wrist is what’s concerning to me.

In my experience with wrist-based heart rate, it’s terrible, except for 24/7 everyday monitoring, which seems to work okay.

I’ve had four Garmin Fenix 5X, three Garmin Fenix 5X Plus, two Garmin Fenix 5S, and two Apple Watches (3 and 4). None of them are reliable for exercise heart rate. Personally, I now ALWAYS ride with a Scosche Rhythm24 on my upper arm (in-between bicep apex and elbow). I have two Rhythm24’s, one downstairs on my trainer bike, and one I keep in my MTB gear bag for outdoor rides. They are both paired to my Fenix.

Two days ago, I was riding MTB, and somehow my Rhythm24 got turned off (never had this happen in 2+ years with a Scosche, could have been from elbow pads I had on). The Fenix 5X Plus HRM took over and my measured heart rate went from 160bpm (Scosche) to 84bpm (Fenix) then bounced around from 110-120bpm while I was gassing out on a climb… then I realized what had happened, got the Scosche turned on again and my HR went back to the 160s. You can see it here on this activity: https://www.trainerroad.com/career/jmvcolorado/rides/54898063-lh-ohv-24-percent-better

The gap at minute 13:00 is where the Scosche turned off, then it took the Fenix a couple minutes to lock a heart rate, then another gap as the Scosche came back online and heart rate went back to normal.

Here’s another example, this time from an indoor trainer ride, with none of the wrist movement or vibration that can throw off an HRM when riding outdoors. The Scosche Rhythm24 in red, Fenix 5X Plus in green. Starts off okay until about minute 15:00. The Fenix is slow to pickup the HR increase in the first interval, not great but not terrible. But after that, it’s completely off the rails. My cool down ended with a HR of around 110bpm, but the Fenix was still reading about 170bpm. Completely useless.


I’ll never buy another Fenix. Just horrible HRM. I have a friend who went to a Doctor because he thought he had heart issues.

Wear a chest strap or an armband.

(Note: my new Apple Watch is WAY more accurate than my Fenix, but still not perfect)

1 Like

I wish an Apple Watch would work for me. But no Ant+ or power meter support is a deal breaker. Plus that battery life… :grimacing:

1 Like

DCRainmaker regularly comments that he gets poor OHRM readings (from a range of devices) while riding outside due to vibration from handlebars, but much better results when using a trainer indoors.

Vibration can cause the watch to bounce, allowing external light to enter the sensor, and give erroneous readings - this applies whether the watch is on your wrist, or on a bar mount (which is why you can get a “pulse” even when it is not on your wrist.).


Totally agree with everything you said. It does have a much better HRM (at least for my body) though.

1 Like

I have a Garmin 935.

The optical HR is spot on. I found that I need to tighten it down one extra notch compared to when I’m not running to ensure a good reading. Also, I shave that part of my wrist. It works! :man_shrugging: About the only time I’ll wear a chest strap is if I’m racing.

Horrible. I always wear a chest strap. I don’t know if it’s the vibrations or hand positions or what, but the HR data is almost always a mess. Indoors on the trainer or outdoors makes no difference, typically there is weirdness. Hence, I always wear a chest strap.

1 Like

I’ve have two heart rate only wrist straps (MIO Link and Fuse) : they can be as good as chest straps.

The most difficult sport to do is Swimming (the MIO Fuse has a memory), but once you have cracked that cycling and running is easy. You can’t wear it where a watch goes though, which is where these built in HRM will fail.

To swim with one I loosen it and rotate it so it looks at my inner arm. Then I push it up my forearm till it is tight. Then I tighten it by 2 positions. The key is to avoid any outside light getting in and that the tissue it is looking at doesn’t move much. Both mine transmit ANT+ and Bluetooth.

The main benefit for me is that you don’t need to remember to put a strap on, they can’t slip and no need to wet the electrodes. First few minutes of swimming always sucks though; think it is due to the blood flow to the skin dropping because of the cold water.

Would need to think about getting a watch type one though. Maybe looking at a watch on your inner forearm would be okay. Would be weird at first.

I’d used a Garmin Vivosmart 4 earlier this year and the HR tracking was substantially inaccurate compared to chest strap during cycling activities. It was OK during all-day. I use a Whoop now and the tracking is accurate, though I’ll use a bicep strap most of the time during rides. I did test on wrist and there were no large variances.

1 Like

My Apple watch is generally within 1-2 bpm of my Wahoo chest strap whenever I glance it it during an indoor ride. I’ve never tried it outdoors though. As others have said, it helps to have the strap tighter than you would for normal wear.

1 Like

Your mileage will vary. Depends on how you wear the wrist sensor (how high/low on the wrist, how tight), your skin (color, hair, …), the sport (vibrations or not, loads on the arms/hands or not), ambient lighting (yes, even that - there can be interference with the sensor), and so on.

In general, for cycling, wrist-based optical sensors are not great. The combo of vibrations and load through the arms and hands create less-than-ideal conditions. Arm optical sensors (such as Scoche, Wahoo TICKR Fit or Polar OH1) and chest sensors are best for cycling.

Wrist based are functional enough to give guidance on your training, which is the limit for which aid use HR data anyway. In the early days I though several bad readings in a session were significant , but they average out. And if you get a prolonged bad reading, you can see it in the graphs or edit it out.

1 Like

The HRM on my vivocactive is ok until I do any proper exercise. Use a chest strap if you want accurate data.

Discussed this very topic with a co-worker a while back. Comparing a Wahoo TickrX to the Garmin Forerunner 935, it seems that (for me) the “trend” was similar, but the wrist sensor lagged during warm-up and cool-down. Images below are from the same easy TR ride.

chest strap HR:

wrist HR:

Chest strap for the win if you care about the data. #HRSS

How are you able to log both wrist and chest heart rate? Does it to it natively on the watch?

The chest hr monitor can be paired to the watch…and unpaired. :wink: Did a TR ride that used the chest monitor and logged to Strava. Recorded an indoor ride on the 935 and logged it to Strava from the Garmin Connect app to compare. Then paired them back up for next time.

So those are two completely different data sets then from two different rides?

I was thinking you were logging both simultaneously.

Edit: ahh I think I get it. You did an indoor TR ride, recorded the chest strap to a phone something and the wrist to the watch. So two separate devices recording the same ride…

1 Like