“You never heard of a FitBit?”
Garbage. I could train in Thai boxing for an hour and it registered almost nothing. Thirty minutes pushing a self-propelled lawnmower said I had been walking seven miles. They use a simple vibration sensor. Not really high tech.
Things like wearables are really only good for people who want to maintain a certain heartrate for a time and want to keep track of that or so they compare with previous performances to see if they did more this time than last time.
For others, who just want to work out hard until they can’t, it’s not the most useful kind of device.
I have a wearable Casio Watch and I will use it on occasion when I’m driving to see to the 1/10th of a kilometre the distance because my car only measures in km and not hm.