How swim tracking works on the Apple Watch Series 2 and 3

"Damn it, Jim, I'm a roller skater, not a swimmer!" — me, more or less

When you swim in open water, your Apple Watch will create a GPS-based swimming map for you to look at afterward. GPS signals don't penetrate water, so the Watch grabs a connection any time your hand is above water mid-stroke. This pitch sounded pretty dicey to me — that's a very short amount of time to properly talk to a satellite — but in my brief 10-minute test, it worked pretty well. (Admittedly, not hard to do, given that I swam straight out, then straight back.) The resulting workout in the Activity app also attempts to analyze your dominant stroke, distance, and pace.All in all, a fun experiment, though I'd love to see professional athletes give this a more thorough test — surely the drag of an Apple Watch wouldn't slow down the lap time of champion swimmer Katie Ledecky.

Apple built its own algorithms to discern the stroke of swimmers of all skill levels, as well the calories they're burning. The process involved gathering data from more than 700 swimmers and over 1,500 swim sessions. And Apple gathered even more data from people swimming in place (in an "endless" pool) while wearing a mask that descended from the ceiling.That's a lot of data. And all that data is required, because skill level affects form. "If you are Michael Phelps, you're distinctively making very good differences between those four strokes—the backstroke, freestyle, breaststroke, and butterfly," says Jay Blahnik, who directs fitness and health technologies at Apple. "If you are me, it turns out that once you look at the signals from the gyroscope and accelerometer, they're not as strong and as clear."

The accelerometer measures motion, and the gyroscope determines how many degrees the watch is rotating per second. Together, those sensors help Apple figure out the stroke type."What we're looking at is literally the trajectory of your wrist as you're doing each stroke," Huang says. Butterfly and freestyle look most similar to the watch's sensors, which presents their biggest "confusion matrix." But it's important to get it right, as stroke type affects how many calories you burn. Butterfly, for example, can burn about 40 percent more calories than breaststroke.To complicate matters, the gyro has a tendency to indicate that the watch is rotating, even when it's not—a phenomenon called gyro drift. For example, your wrist could be still, but the gyro might say it's rotating by 10 degrees per second. In this instance, Apple relies on the accelerometer to find the ground truth. If it sees from the accelerometer that you're not moving, but the gyro still shows rotation, it can learn by what amount to discount the gyro's reading.