The Fitness Wearables Data You Can — and Can’t — Trust

When you strap on a fitness wearable, you suddenly have a wealth of data about your own body. The paradox of wearables is that there will always be a mismatch between what you want to know, and what a bundle of sensors strapped to your wrist can actually provide. Fortunately, scientists can compare the output of wearables to known standards for measuring things. They can check how accurate a given device is at gauging your heart rate, measuring sleep stages, or estimating the number of calories burned during exercise. Unfortunately, those studies don’t happen before the devices are released. So, with that in mind, here are some things that you need to watch out for when selecting a fitness wearable. 

  1. 1. Pay attention to sleep time, but not stages. When you’re asleep, you stop moving, and your heart rate and other measurements like skin temperature can help the device tell the difference between actually sleeping versus reading in bed. Wearables are not good at telling the difference between stages of sleep. So, trust the amount of sleep it says you got, but don’t trust the sleep stages or sleep quality scores. 
  2. 2. Trust your heart rate during exercise, but not zones. Heart rate is a relatively simple measurement: Your heart is either beating at this exact moment, or you’re between beats. While it’s reasonable to expect this metric to be relatively accurate, devices tend to have trouble getting an accurate heart rate when you’re moving around a lot. If you’re training with heart rate, you’re probably using zones. Zone 1 means you’re barely working, while zone 5 is an all-out effort that you can only keep up for a few seconds. Zones are based on assumptions about your heart, usually calculating your maximum heart rate based on your age, and those calculations are often very wrong. 
  3. 3. Trust your calorie burn, but only in the big picture. The biggest feature of wearables for most people is the fact they can tell you how many calories you’re actually burning each day. Too bad they aren’t accurate. The calorie burn numbers can be useful in the big picture, but don’t nickel-and-dime yourself when it comes to specific numbers. Trust the general trend, using it as a reality check on whether your activity levels have gone up or down. Don’t trust the exact number, especially for individual exercise sessions. Fueling your body and feeling good are more important than making the numbers match up exactly.