I saw a gif online of someone holding what looked like an Apple watch against the skin of a banana and it registering a pulse. I’ve tried it with my Fitbit Versa and got the same result, it showed a pulse of 70 bpm. What’s going on here?
It’s picking up the heartbeat of the person. Place it on the banana and let go. If it still registers a heartbeat, don’t eat the banana.
It is registering the movement of the baby wandering spiders.
Or sometimes this:
Banana with a Pulse, sounds like the title of a Harlan Ellison story.
Banana Worms
I assume your question is tongue in cheek and you’re not serious, but in case you are: I struggle to understand how, after extensive testing, Apple received permission from the FDA (and many other countries) to use Apple watches as a valid ECG device if it’s heart monitoring sensor of so screwed up it measures the pulse on a banana.
Perhaps like pretty much all gif’s, meme’s etc this one was a joke? 
Before you break out the smug “how could you be so dumb” rolleyes you might want to read the link I provided.
OP’s Post:
what looked like an Apple watch against the skin of a banana and it registering a pulse.
I did, maybe you should have a closer look too.
Read your own post:
the less sophisticated fitness trackers
The Apple Watch Series 4 managed to cram in an EKG scanner, which measures electrical signals instead of light — a godsend for people with chronic heart conditions.
After reading the link, I have become convinced that I need a fitbit and a teddy bear. I know JUST the person to freak out with a stuffed toy showing a pulse!
Which means that the Apple Watch Series 1 through Series 3 did not have the EKG. And the OP said nothing about “I saw a gif of an Apple Watch Series 4 or beyond picking up a pulse on a banana.” but feel free to keep digging your hole.
I missed the part in the OP where he said it was a series 1 to 3 watch My mistake.
You know, that doesn’t really even matter, because it works with EKG-based Series 4 too.
The EKG in the Series 4+ is a separate sensor. The optical heart rate sensor is still there, and that’s what they’re using on the banana.
I’ve worked on this technology. Optical heart rate sensors are very hard to get to work reliably - they have to deal with different skin colors, tattoos, people wearing the watch too loose, people wearing the watch too tight, motion artifact, stroboscopic effects from high-frequency LED lighting, sunlight, tendons moving in your wrist when you’re using your fingers, and all manner of other confounding factors.
You can design the algorithms to try to reject false signals, and all these products do (especially Apple) - but this is also hard, because you’re invariably going to throw away some good data too, so you can’t be too aggressive about it or you’ll make the product useless for people who are exercising, people with dark skin, etc.
Most of these confounding factors can change very rapidly as the watch shifts on your wrist or you move around - so even if your algorithm thinks it has a bad signal for a few seconds, it needs to re-evaluate the next few seconds independently. If you’re moving around a lot, the watch might only get a good signal for a few seconds, every few minutes.
If there is no actual biological signal to be measured, it will occasionally lock in on whatever statistical noise is present that might temporarily resemble the heartbeat signal it’s looking for, and then display that as the current reading up to some timeout value (minutes), after which it’s considered too stale.
The Apple Watch does have a wrist detection algorithm, and if you’re just casually wearing it around the house, it will do a pretty good job of detecting that you took it off and it will stop recording your HR in the background. However, if you open the HR app and deliberately put the watch into heart rate mode, as is done in these videos, it bypasses those algorithms and assumes you are actually using it on your wrist and trying to get a reading.
Bottom line: don’t rely on these devices for anything too critical! They’re good for fitness tracking, that’s about it. The ECG on the Apple Watch is more inherently reliable but the algorithm can also be fooled in a variety of ways (and ECG requires that you touch the watch with your opposite hand in order to get a reading).
Maybe the banana is just excited to see you.
What’s the banana has in it’s pocketses?
A banana with a pulse? I think I worked for him many years ago.
A banana and a pulse walk into a bar. The banana says “Hey, man, where you been?” The pulse says “I bean everyhere you moron!”