What causes the wrist-based pulse spike at the beginning of run-off-the-bike?

When executing a brick, pulse measured at the wrist spikes for the first 4/10ths of a mile then settles in to a more realistic measurement. Here are three different athletes with similar janky readings at the beginning of their run…

Any thoughts on what causes this?

Wrist based HR measurement! :rofl: sorry I’m wearing my captain obvious hat right now lol.

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Calling @dcrainmaker for an opinion on wrist based HR jankiness at the start of a brick run.

What watch is it and yes @dcrainmaker is the man for this.

I recently purchased the garmin Fenix 7x, it was pretty expensive for me for a watch. The heart rate readings are very inline with my heart strap linked to the garmin 530,
it’s kind of insane how accurate it’s been.
I had a garmin vivoactiive3 before this and the heart rate readings were all over the place.

Try harder. What about wrist based pulse causes the spike & settle pattern.

Can’t see the HR, but putting my degree/specialization hat on, maybe it’s a temporary false lock on foot falls / running pace. Kinda looks like 2x HR is the false lock. So maybe another type of harmonic on the real HR? I dunno the pics lack detail.

Hmmm…that’s a good hypothesis. I notice, too, that it looks like most of these are athletes that recorded the swim on the watch, the bike on the headset, and the run back on the watch. So maybe that’s part of it.

I read up on the PPG technology a long-time ago, thought I had a paper or two but can’t find them. Did a quick internet search and came across this:

which has some info on false lock on running cadence.

And this:

from a maker of sensors used by some companies. Note the following:

“If you’ve seen the green blinking lights on the back of a smartwatch or fitness band, that’s probably utilizing at least one of Valencell’s inventions. Those lights are shining light into the body, and the sensor is capturing blood flow patterns within the scattered light. The challenge is that these patterns cannot be extracted in a simple, linear fashion. To address this limitation, Valencell has developed a “Deep-PPG” neural network that is capable of analyzing these patterns to determine biometrics such as blood pressure, heart rate, heart rate variability, blood oxygenation, respiration rate, and more.”

As mentioned in the Garmin article, and above, those blood flow patterns in the wrist are complex, subject to interference, and challenging to extract.

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I’d be curious too if the switch from standby/watch mode into activity mode has an impact on the HR measurement - I believe it does ramp up the power delivery to the sensor and likely the sampling rate. I wonder if since they’re not actively recording while on the bike, when they fire it up to record the run it takes a few minutes to develop an accurate measurement. Completely different tech and unrelated, I realize, but in a similar way that it can take GPS a few minutes to develop a lock.

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Get them to dual record HR with a chest strap too?

My watch has issues with sudden jumps in HR too, but usually it reads too low. For example, at the start of a cross race (basically a max power sprint into vo2max for as long as I can hold it, lol), I’d expect a spike. But my watch usually stays at “low endurance” level for about 8 minutes.
I think it sort of expects HR within “zones”, and needs a bit to realise HR is actually higher or lower than the current zone.

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