Strange behaviour with Garmin HRM

Francesco Pagano

I ran a new test today and something interesting came up.
I had the HRM chest strap connected to both my S9PP and to an old Garmin Vivofit watch so I could see my current HR on both. I started an easy run and at some point the Suunto watch again spiked from a reasonable ~150 bpm to over 170 whilst the other watch was displaying the expected HR, which corresponded to what S9PP on the pause screen.
This suggests me that Suunto watch is displaying a different value than what the chest strap is sending.
The funny thing is that this pretty much always happens at the same point, when I run a very short hill down and my pace and cadence increase for a few seconds. Today and previously it seemed that my “fake” HR corresponded to my cadence, so could this be a case of cadence lock?
In other cases when the displayed HR was way too low I can’t see any relation instead.

@Francesco-Pagano I’ve not heard of cadence lock with HRM pro…

isazi

@Francesco-Pagano while cadence lock is something that can happen with optical HR (because hitting the ground can create artifacts for an optical reader), I cannot imagine how this could happen with an electrode. Unfortunately I have not used a Garmin HR chest strap since 2019 so I do not know what is transmitted, if heart beats or inter-beat time (although I guess the latter).

Mff73

@Francesco-Pagano
Are you sure that your S9PP locked on the belt HR before starting the activity?
If yes, your tests may lead to think that your watch is using wrist HR despite what you think. But how to prove this

TonyG

@Francesco-Pagano

I had these issues all the time with 9 peak & suunto 9 baro, and did loads of experiments to try and work out what was happening but more importantly how to resolve or stop the issue happening in the first place.

https://forum.suunto.com/post/114542

ODo67

Hello!
I’ve had two Hrms. Neither lasted more than 6000 km (1 year and a half for me ️). I don’t think they’re designed to last any longer, especially with the non-replaceable strap. Since I switched to Suunto for my watch, I’ve bought a belt that costs half as much and has a replaceable strap!

false

Does anyone know here how it works under the hood? I mean the belt transmits ready to use value of HR or the watch does some calculations before showing the data on the screen?

Francesco Pagano

@Mff73 Yes I only use the belt and I would know if I am starting an activity using OHR instead. As I wrote previously, am I even wearing the watch on my jacket sleeve so I wouldn’t expect HR readings from my wrist.

Francesco Pagano

@TonyG Thanks for the advice, I’ll try the strap position you suggested there. Today it happened again but luckily the HR went back to normal in a few seconds after I paused and restarted the activity.

isazi

@false I can only answer for the Suunto sensor, although I believe most chest straps work the same: the sensor sends IBI to the watch (the time in milliseconds between two beats) and the watch uses that to compute heart rate (watches may use different algorithms, especially on the filter side, to do that)

Francesco Pagano

After some time I am more and more convinced that this problem is related with wearing a jacket.
When running with just a synthetic t-shirt on warm days I haven’t experienced it. Yesterday I ran again with t-shirt plus jacket and the HR values were all over the place. I did some research, and it seems to be a known issue.
It’s just strange that the HRM is sending correct values, as displayed both on the Garmin step counter and on the S9PP on the pause screen, whilst the wrong values are displayed (and recorded) on S9PP during the activity.

Mff73

@Francesco-Pagano
maybe a data filter process difference between activity pause and activity recording ? Only Suunto expert in FW could say. It would not be logic, but who knows.