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Garmin FR 235 Optical HR Sensor Testing : HRV ?
Posté le 11 décembre 2015 à 11:25:46 par gpspassion.

HRV Testing


Garmin HRM3 - Garmin FR235 - Polar H7

The recently released Garmin Forerunner 235 has attracted quite a bit of interest due to its "in house" built-in optical HR sensor as per this article on the DC Rainmaker website but also because of the (apparently) HRV derived data Garmin provide without an external chest strap, i.e. VO2Max, Training Effort (TE) and Recovery.

Up to now there were two main players in this field, Mio Global and Valencell/PerformTek, the former being used on the Mio bands (Link, Alpha, Fuse), the Garmin FR225 and the TomTom Cardio (not the Spark) and the latter on the Scosche Armband. Both these sensor solutions have given the best results for sports of the optical solutions out there as shown in the comprehensive reviews on the DC Rainmaker website but they were somewhat frustrating to sports aficionados because they lacked the ability to provide precise "r-r" data required to calculate HRV, unlike traditional but "cumbersome" chest straps (see this comparison by Marco Altini). The finer points of HRV are out of the scope of this topic but to sum it up, it's a technique to measure the "state" of the heart and adapt the training schedule to stay optimally trained as overtraining is often "around the corner".

What's confusing is that Garmin watches (FR620, FR920, Fenix 3) that provide HRV derived data (VO2Max, TE, Recovery) did show it when paired over ANT+ to a Scosche or Mio band but that's because "r-r" intervals have to be provided as part of the ANT+ specs as pointed out on the DCRainmaker site, however it was just "estimated". With the FR235, Garmin initially said (it's in the manual actually) that VO2Max would require the use of a chest strap but DCRainmaker reported that Garmin were saying they were working on getting VO2Max readings from the built-in sensor. Indeed FR235 users found that they were indeed getting VO2Max, TE and Recovery and assumed that Garmin had found a way to coax out precise HRV data out of there in-house sensor. With the "basic" (simple hearbeat) accuracy of the FR235 HR being "so so" it seemed too good to be true, so I set out to measure the "r-r" data provided by the FR235.

Setup :
- simultaneous recording of 60 second "r-r" data from the FR235 in ANT+ "broadcast mode" on my laptop and from the Polar H7 chest strap over Bluetooth on my Android smartphone
- recording a few minutes later of 60 second "r-r" data from the Garmin HRM chest strap over ANT+ on my laptop to validate the recording technique. It wasn't possible to record the H7 data at the same time due to interference of the two straps on my chest.

Results :

- FR235 vs H7 "r-r" intervals in milliseconds :

- Garmin FR235 data interpreted by gHRV :

- Polar H7 data interpreted by gHRV :

For validation purposes, Garmin HRM recording interpreted by gHRV (click for larger picture):

Conclusion : There isn't a lot to add to what the screenshots show quite clearly, that the FR235 (like the Mio Optical Sensor) doesn't provide proper "r-r" data (or very accurate BPM data for that matter but that's for another thread) unlike the Polar H7 and Garmin HR chest straps. As a result, the VO2Max, TE and Recovery metrics provided by the FR235 cannot be compared to the metrics obtained from a chest strap. This is disappointing although to be fair to Garmin they never claimed that they were getting precise "r-r" data out of their sensor, it was somewhat implied though...At the end of the day these metrics are "estimates" so one could take the view that they're just a bit more "estimated" when coming from an optical sensor.

If you have questions or comments you can use this topic of the Sports GPS Systems forums.

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