Wearable force estimates are guesses dressed up as measurements

A watch reports peak force to one decimal place, and almost none of that precision is real.

Two decimal places on a screen is a claim. It says somebody measured this, and in the case of most running wearables nobody did.

What the device has is an accelerometer strapped somewhere that moves relative to the bone underneath it, plus a model. The model takes acceleration, the mass you typed in when you set the thing up, and some assumptions about how a human runs, and it produces a force figure. That figure is a calculation from a guess about your body, and it is presented in the same font as your heart rate, which was actually measured. The interface makes no distinction. The athlete cannot make one either.

I am not being purist for its own sake. The problem shows up in decisions. A runner sees estimated impact load creeping up and cuts volume, when the change was a new pair of shoes altering how the watch rattles on the wrist. Or a coach compares two athletes' figures directly, which the model cannot support at all, since it is calibrated on population averages and each of those athletes departs from the average in different directions.

Where the devices earn their keep is within one athlete, over time, on the same surface, in the same shoes. Under those conditions the systematic error is roughly constant and the trend line means something even though the absolute value does not. That is a real use and I would not want to lose it. It is also a much smaller claim than the marketing makes.

The interesting question is why anyone wants a force number in the first place. Usually it is a stand in for injury risk, and the honest position is that the relationship between measured tissue load and injury is weak enough that even proper laboratory force plates cannot predict who is going to break. Adding an estimation layer on top of a relationship that fragile does not produce insight. It produces confidence, which is worse than nothing.

Use the wearable for the things it genuinely observes. Time. Distance. Heart rate, with reservations. Cadence, which is just counting.

Everything else on that screen is a model with a marketing department, and you should ask it for its working before you let it plan your week.