This is scientific American 60 second science. I'm Christopher and Tagyatta.

Fitness bands like the Apple Watch and the Fitbit aim to track your vitals, like heart rate.

But early models weren't all that accurate.

We thought of them a little bit like random number generators. They really didn't seem to be providing

anything that bore any relationship to heart rate.

You and Ashley, a cardiologist who studies

wearables at Stanford University. He and his colleagues have now tested

seven newer fitness bands from brands like Apple, Fitbit, and others and he says

those heart rate stats have gotten way better.

Yeah, we were pleasantly surprised actually by how good the accuracy of the heart rate monitoring

was.

For most of the devices, the air rate was less than 5%. That's good enough for your doctor. But where all the devices

failed to measure up was estimating calories burned. Even the most accurate devices were off by

27% compared to lab measurements of energy expenditure.

One device was off by more than 90%.

If you think about going to the gym and working out for an hour and maybe that's around 400 calories

then in reality that could be anything from 200 to 800.

And that's a big difference if you're thinking about somebody who's incorporating those estimates into their lifestyle and in particular thinking about what to eat that evening based on the workout they did that afternoon.

The results are in the Journal of Personalized Medicine.

The reason for the discrepancy Ashley says could be that we all burn energy at different

rates and that's hard to reckon from simple input stats like weight and height.

Some people are incredibly efficient and look incredibly elegant when they run and others really clearly look like

...