I am trying to understand heart rate readings in fit-files and had a look in a recent swim activity using Garmin Fenix 6S + HRM Pro plus
When I open the fit-file I find under the heading Record timestamps with corresponding heart_rate (bpm). The bpm is quite high.
However, in Intervals.icu under Interval Data, I can download a streams.csv, where I find the variables time,cadence,heartrate,distance,velocity_smooth. Here, heart rate values are considerably lower than in the fit-file mentioned above. It appears that Intervals.icu use these heart rate values for charts and other downstream outcomes.
Are these heart rates from the two differents sources Fenix vs HRM pro plus, and if so, which one is which?
The heart rate values in streams.csv should be the ones from the file. However Intervals.icu generates streams from the length data for swims and only keeps heart rate data overlapping that range. So the first few heart rate records seen in the fit file are likely discarded because you hadn’t started lengths yet.
Thank you for your rapid response! I still do not get it:
The time-stamps + heart rate are identical in streams.csv and downloaded fit-file, whereas in the original fit-file (at identical timestamps) the heart rate are considerable higher .
So, what on earth does the heart_rate (bpm) in the original fit-file represent? And what is it used for?
What is under the device_info header?
Only your watch, or something more?
My watch and an antplus device, presumably my cheststrap HRM. The number of rows differ between activities, but here is an example:
Hmm … I have had a closer look and something isn’t right. I will look at it some more on Saturday and hopefully I can figure it out. Tx for reporting this.
There are a bunch of HR messages at the end of the file that are not displayed by fitfileviewer.com. These contain different HR data. Each message has several filtered_bpm values. Intervals.icu uses a standard FIT SDK plugin to replace the HR data in the record messages with this data. As far as I know that is the correct approach.
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I’m not sure I understand what you mean, 
. Irrespective, I am thankful for your fast and helpful response and have decided to become a paying subscriber!
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Thanks. Basically there is a bunch of extra HR data added to the end of the fit file for swims. Apps are supposed to use that instead of whatever is in the records.
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