1-Wrist heart rate:
a)
Reliability: Employs an optical technology to read heart rate. Quality of the signal depends upon correct placing (below ulnar styloid: the prominent bone of the wrist), proper tightness to the wrist and (lack of movement), which highly affects correct reading (the reason why it is so useless for sports like running) and light (but they are prepared to discern light waves and even operate in dark conditions).
b)
Limitations for freediving: Vasoconstriction of peripheral limbs during breath hold limits bloodflow and therefore affect wrist optical reading. This could affect static wet apnea reliability, although I did not found this problem when performing dry static apnea. During "moving modalities" (dynamic and deep immersion) this might not be so affected (when using arms for swimming blood flow is not so impaired), but depends on subject's diving response, temperature and probably wrist capilarisation.
c)
Usefulness for freediving: Wrist hear rate is
the only alternative I know to provide instant heart rate feedback and the only alternative to outdoor (sea deep dives) modalities. The optical sensor is part of the sportwatch, so they can provide instant readings and as far as the watch can get underwater. You will need to keep the sensor in contact with your wrist (adjusting your sleeve wetsuit and glove) with the screen visible to you and configure the watch to always have the backlight on if neccesary. Keep in mind that if you operate the buttons at high pressure under water (when outdoor sea deep dives) you risk water entering your watch and damaging the device. I have not employed it for dynamic apnea, but I fear it won't be as reliable due to the movement, yet being a comfortable option (no need of extra gadgets).
d)
My experience: I find this feature useful and comfortable to provide information about my relaxation state before any immersion, keeping in mind it might not be accurate, so I always double check by finger contact with my neck vein (if my pulse is slower than seconds, it should display HR below 60 bpm). I can gather data from outdoor dives to analyse afterwards (in my computer) and I really like watching my HR before and after a static apnea (and how it fluctuates during it). Since I am a multi-sport guy, I use a Garmin Fenix 5x plus sportwatch (which can be submerged underwater up to 100 m---be careful since testing of these devices are in non-motion) with a third-party app for deep dives (
Apnea CWT which has a cost of 12€, and
Apnea DYN by same devoloper but free). This app allows me to see data (depth, speed and heart rate) in my computer after uploading the file to their online webpage.
Here is an example.
2-Chest hear rate monitors on the other hand:
a)
Reliability: Much more reliable since they collect electromagnetic waves directly from your heart, overcoming the vasoconstriction. You will need a chest hear rate band which can operate underwater which then comunicates with a sportwatch which records and dump the data.
b)
Limitations for freediving: To the best of my knowledge, there isn't a chest heart rate band which can tollerate the high pressure of deep dives. They are usually IP67 and could be used for swimming or even dynamic apnea up to the specified 3 m depth. Furthermore, either ANT+ or bluetooth communication do not work underwater, so heart rate is stored inside the HR band (at least with the Garming TRI and Garming swimm that I use) and dumped after the activity, but can't show instant heart rate while underwater. It can display instant HR as far as the chest band and sportwatch are off water.
c)
Usefulness for freediving: I would say they are the
most reliable devices to record sallow indoor (swimming pool) modalities, with limited ability to display instant readings. It won't display HR while face down during a static apnea (though it will be recorded), or while swimming in any dynamic (or during preparation since your chest is submerged in vertical position) but they overcome vasoconstriction and provide good quality, reliable readings to be recalled after a workout.
It should be the preferred method also for dry training (dry static and any sport involving movement like apnea walks/run/cycling) due to its reliability. Be cancelous if you plan to use it in deep dives since chest bands are not prepared for high pressure!
I hope this information helps you.