I have two watches on my left wrist, another on my right arm, a ring on my finger and a sensor embedded in my bra. No one should ever wear this many fitness trackers simultaneously. But in this moment, I am letting the latest heart-rate-sensing, sleep-capturing, workout-recording wearables from Apple, Fitbit,
Whoop and Oura capture all my data, to see which ones do the best job.
Whether you are training for a race or trying to lose the Quarantine 15, a fitness tracker can provide a helpful motivational nudge. But choosing the right one depends on your preferred activities and health goals—and requires an understanding of what data is most useful to you.
New trackers are much more than pedometers and heart-rate monitors. They also measure blood-oxygen levels, a sign of overall health and altitude acclimation. And they capture heart-rate variability, the difference in time between each heartbeat, which can provide feedback about your body’s ability to recover from exercise. But how accurate are the metrics?
The Right Metrics
Studies have shown that wrist-based wearables’ optical sensors—which beam light onto the skin to detect pulse—are generally accurate during rest, but less so during workouts with unpredictable movement, such as strength training.
The data can still be valuable. Zakkoyya Lewis-Trammell, an assistant professor of kinesiology at California State Polytechnic University, Pomona, said that while wrist-based trackers aren’t clinical devices, she considers them a reliable tool for comparing day-to-day changes.
One helpful approach: Pick metrics that are as close to the sensor’s data as possible—such as resting heart rate over time, said Seth Martin, a cardiologist at Johns Hopkins Medicine. Avoid obsessing over things like calories burned, which is just a calculated guess, he added.
A new metric popping up in wearables looks at whether your body is primed to work out. Fitbit and Oura call it “Readiness.” On Garmin, it’s “Body Battery.” Whoop assigns you a “Strain” score. Several Apple Watch apps, including Training Today, offer similar measurements. A high score means you’re ready for intense exercise; a low score indicates your body needs rest.
It isn’t as valid for people who have diabetes, heart disease or who are pregnant, Dr. Lewis-Trammell said. In those cases, she said, the data should be brought to a physician for evaluation.
For people who do outdoor workouts, GPS accuracy is important for logging distance. Dr. Lewis-Trammell found Garmin devices have a better GPS than others. I confirmed this in my own testing.
On one ride testing the wearables’ GPS-tracking capabilities, the Apple Watch and Garmin tracks were very close to my actual route. The Fitbit didn’t lock a GPS signal until about half a mile into my workout, so it displayed less total mileage. It also cut corners (literally) and showed me riding through neighbors’