IN DEPTH: The Quest For Wet HRV – Swimming HRV Underwater – Finally

I started off a couple of years ago with the intention to record ‘every heartbeat’ that I do in exercise. Not because I like collecting things but rather to get a more holistic view of the Training Load (TL) of my triathlon efforts. With a better view of my training load I could, for example, better plan to rest and/or train harder.

I naively imagined it would be straightforward.

It’s not been easy. Indeed it’s become the much-fabled “Quest for Wet HRV”.


As time progressed it transpired that some form of Heart Rate Variability (HRV) would be a further valuable insight into the area of post-exercise recovery and adaptation. So my intention became, instead, a quest to record every triathlon-based RR ‘beat’ – RR ‘beats’ are what are used for HRV.

Again, not rocket science. Garmin’s original 19th Century hard plastic chest strap is certainly waterproof and fully able to transmit RR beats.

Because of all this ‘5krunner stuff’ that I do, it only seemed fair to try to achieve this on all major hardware platforms ie Suunto, Garmin and Polar.

STRYD: Garmin 920XT, Suunto AMBIT 3 SPORT, Polar V800
STRYD: Garmin 920XT, Suunto AMBIT 3 SPORT, Polar V800

A further constraint is my belief in the data openness that will enable us all to review and analyse our training data wherever we want. So, whilst I can’t test every 3rd party data/analysis platform, I thought it would be reasonable to be able to ‘do it’ in SportTracks – it’s a fairly open piece of software and cheaper than a similarish TrainingPeaks package and also has a rather nice implementation of Training Load.


Yet another constraint is to get data into the COACH function of Firstbeat’s ATHLETE software for RR/HRV (discontinued but working software) as this software has a nice feature that dynamically suggests the intensity/duration of your next session.

As you can see, an initially straightforward task morphed into something a little more complex but by no means unreasonable.

Anyway back to the reality of February 2016.

Cycling & Running

Many chest straps are capable of recording RR beats when out of water. Polar, Garmin and Suunto all come up trumps there. We don’t really have to say much more than that. If you run, cycle or do duathlons you don’t really have much to worry about.

In my case sporttracks imports all their files and Firstbeat Athlete links directly to Suunto Movescount as well as importing Polar’s HRM and Garmin FIT files.


Swimming is the bête noire of HR recording as ANT+ and Bluetooth signals don’t travel more than an inch or so underwater. The solutions to this are 1. ignore it (Garmin pre-2016) 2. Use a different communication frequency (Polar) 3. Cache the data in a HR strap (Suunto)

a. Suunto: with Suunto’s underwater, caching Memory Belt on the AMBIT3 everything I dreamed of is possible. Indeed way back in 2015 and probably 2014 it was also possible.

Day 2: Power vs Pace
Day 2: Power vs Pace

Sporttracks accepts their sml files and Firstbeat ATHLETE connects automatically with Suunto MOVESCOUNT and/or can directly import Suunto files such as SDF.

The Suunto MOVESCOUNT environment also mostly does what I would want it to in the context of this article around HR/HRV analysis of fatigue.

b. Polar: with a H7 HRM and a V800 we’re mostly good-to-go. Even though the H7 is Bluetooth it also transmits on a non-Bluetooth frequency though water for the V800 to receive as you swim – there is no need for caching.

4iiii Viiiva V100 Heart Rate Monitor, Polar H7, Wahoo TICKR-X, Suunto SMART Belt, Garmin HRM-RUN
4iiii Viiiva V100 Heart Rate Monitor, Polar H7, Wahoo TICKR-X, Suunto SMART Belt (the round one!), Garmin HRM-RUN

SportTracks imports the .HRM files as does Firstbeat ATHLETE (usually).

The Polar environment also does a great job of analysing training load and recovery. Better than Garmin and Suunto.

c. Garmin: Pre-2016 those of you in the Garmin environment were forced to try strange things with other vendors’ hardware (MIO – won’t store this non-HRV data, Suunto – the app didn’t work without a watch but does now, WAHOO – app doesn’t transmit HRV). For example I used to use the excellent WAHOO TICKR-X (caching) but getting the data back to SPORTTRACKS is tricky and not possible (HRV) to ATHLETE. So I only messed around with the process occasionally to validate the estimates I otherwise used.

Garmin HRM-TRI (HRM-SWIM) Review
Garmin HRM-TRI (HRM-SWIM) Review

Garmin’s late-2015 HRM-TRI and HRM-SWIM are caching HR straps and these promised a solution. However they do not (Feb 2016) follow a published ANT+ standard and so are not properly open to ATHLETE and FIRSTBEAT (and others). Sigh.

However the clever people at SPORTTRACKS have now deciphered the non-standard data.

Yes Result!

Err no.

Of course the non-standard FIT files from the HRM-TRI are not compatible with FIRSTBEAT ATHLETE. Of course that software is no longer supported and hence no longer developed and so will not be modified 🙁

There are over 1000 posts on this blog and I don’t think any swear word is in any of them. Until now….””$”$%^£%&%^*^&((.

As always there is a clever person somewhere. It’s just finding them that’s hard. The FIT Repair Tool does decipher the non-standard FIT file and then can save it as a Suunto SDF file. This can then be imported to ATHLETE. TA DA !!

So after two years I have completed my quest for the Holy Grail of Wet HRV.

Half Time Summary

So now I can do what I originally intended to.

Of course part-way through the quest I realised it was flawed. Here’s why

  1. Female triathletes will be super happy with costumes that cover straps. But male pants-only triathletes won’t want to look like an idiot and wear a HR strap in a pool. No HR data. Or you could wear a wetsuit and look equally as ridiculous. There is no wrist-based HRV alternative **YET** (the quest continues).
  2. Firstbeat ATHLETE has no concept of different zones per sport. I’m not exactly sure how their TRAINING COACH function works but, in part, it looks at the TRAINING EFFECT of time in zone. so it underestimates swim efforts and also cycling ones albeit to a lesser degree (for me).
  3. It is possible to change the HRmax for each session manually in ATHLETE (that is the ‘solution’ – ie make HRmax SWIM lower)
  4. Even if proper zones can be determined (as with SportTracks) then who is to say how much the training load of one sport impacts on the fatigue state of another sport? Will a TE4 swim one day prevent me from executing a TE 4 run the next? In my case it wouldn’t. But the stats disagree at that level of granularity. Sigh.
  5. Even if all the TRAINING LOAD stuff were correct then will I be able to execute a TE4.2 bike today? (I am about to). The stats say yes. However if I only got 2 hours sleep last night then reality may say otherwise.
  6. That’s why I also use WAKING HRV as an input to my readiness.

The quest is over.


I have 3 further LONGTERM distractions on this quest which I don’t think can be properly met yet:

  1. To record every step – In Douglas Adams’s style I’ve decided this is ‘mostly pointless’ for anyone vaguely sporty, although I would still like to do it if I could find an activity tracker that I would not be embarrassed to wear. Polar A360 looks promising.
  2. To record every beat/RR-beat whilst sleeping – this is probably a questette too far. Essentially though it is a good one as a good night’s sleep is probably the best recovery any of us can undertake. Measuring and analysing that could give us more insight on how ready to train we are the following day. However this needs a good bit of wrist hardware and decent analytical software. I can’t see that happening in 2016 (EDIT: wrong…QS EMFIT does the job!).
  3. Have a single, aesthetically pleasing wrist-based device that can do all the above, probably using optical Heart Rate Technology.

The single, open, beautiful, wrist-based solution is just pie-in-the-sky and I’m not holding my breath.

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4 thoughts on “IN DEPTH: The Quest For Wet HRV – Swimming HRV Underwater – Finally

    1. hmm very interesting. that’s a new one to me. I have a few options for night hrv that I am looking into but that wasn’t one of them!! might even work for swimming if waterproof

      1. HRM-Pro v7.40 records Swim R-R, and syncs to FR945, only FitFileRepairTool reads the Swim R-R from the resulting Fit file.

      2. Nov2021 Update !
        thank you. MY understanding of the science has changed and that impacts on what I do with tech.

        I just take waking HRV readings as a measurement of readiness.
        I still do capture basic HR for all workouts.
        I do NOT use CP/FTP power for training load partly because the calcs have some inbuilt omissions/errors.
        i use H10+HRV4Training for waking hrv, also Oura and Apple Watch.

        the next leap forwards with hrv will be around DFA a1 and uses at higher levels of exertion. There is already a place for that and some people use it. I don’t think it will help me so much right now but it’s valid to use.

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