HR Source Switching - what is the point?

S.M.R. said:

Sometimes the HRM drops out loosing connection and then this fills in 

Actually, dynamic source switching doesn't handle dropouts (lost connections between the watch and the strap), it handles (allegedly) bad data from the chest strap, which is why a supported Garmin chest strap is required for source switching. (Some ppl, including myself, have found that source switching sometimes prefers seemingly worse data from the wrist over better data from the strap, but usually only for a few seconds, which is why ppl want to disable source switching in the first place.)

https://support.garmin.com/en-CA/?faq=Nf8r6ApX4d9lX0G0flEsVA

Dynamic source switching is available for all activity types to provide seamless heart rate data if the heart rate strap detects low signal quality.

(Here, signal refers to the "HR signal" that the strap reads from your body, not the "ANT+/bluetooth signal" between the watch and strap. This is made clear by the following text)

The biggest benefit of the feature is for running activities. A heart rate strap is usually the most accurate source of heart rate data for running but occasionally there can be inaccurate heart rate at the beginning of runs, often referred to as start-up noise.  This is caused by poor connection between the skin and the electrodes and typically resolves once you start to sweat. With dynamic source switching, when a heart rate strap detects low signal quality, heart rate from the optical heart rate sensor on the watch is displayed and logged instead. The heart rate strap remains connected to the watch and the heart rate source will automatically switch back to the strap once the signal quality improves.

Regardless of whether source switching is enabled or disabled, if you run with a chest strap (of any kind) and the chest strap loses the connection with the watch, the watch will still use the wrist HR to fill in the gaps (unless the wrist HR is disabled). Garmin watches have done this long before dynamic source switching ever existed.

If all dynamic source switching did was fill in the gaps created HRM dropouts, then there would be no reason for users to complain about the feature or disable it. And there would be no need for a compatible Garmin chest strap to be used.

CitizenX said:

When I pair a chest HRM, it's because it is more accurate, period

Garmin thinks that sometimes the wrist HR is more accurate. While this may be true, it's debatable whether dynamic source switching is able to consistently and accurately figure out exactly when that's the case.

For example, with dynamic source switching enabled and I do a running interval workout, I've seen [*] the watch select the wrist HR over chest HR , as my HR quickly rises when I start a work interval after resting long enough to bring my HR down. In this case, the chest strap clearly responds faster than the wrist HR (meaning that it shows my HR rising more quickly, while the wrist HR lags behind). To me it's not plausible that the wrist HR is correct here, when it shows basically the same data but with a lag of a few seconds, unless the chest HR is somehow able to predict the future.

[*] (after the fact) see next comment for how this is possible

I will also say that the one benefit / side-effect of dynamic source switching is that when it's enabled, the watch will record 3 HR metrics:

- heart rate (the standard activity HR that's always recorded, based on whatever source the watch decides to use at any given point in time)

- wrist heart rate (self-explanatory)

- external heart rate (self-explanatory)

This happens whether or not you use a Garmin chest strap that supports dynamic source switching, and it's a nice way of seeing how your chest strap readings differ from the wrist HR readings. Unfortunately, if you use a supported Garmin chest strap, it also means Garmin gets to decide to arbitrarily switch to the wrist HR whenever it feels like it.

You do have to use a 3rd party site to see wrist heart rate and external heart rate, as they're not exposed in Connect.

1) Open Connect website

2) Navigate to activity

3) Click gear icon (upper right) > Export File. This will export the original activity FIT file in a ZIP

4) Open www.fitfileviewer.com

5) Open the ZIP file you just downloaded

6) Click on the chart icon to graph any 2 metrics you want. (e.g. you can graph any 2 of heart rate, wrist heart rate, and external heart rate, to determine when source switching occurred and to see how your wrist HR and chest strap HR differed during the activity)