How does your Apple Watch know what stroke you are doing?

We love tech that is smart and functional so the Apple Watch and it's ability to nut out your swim stroke automatically is worthy of a deep dive. Calling it a multi-function watch doesn't quite get there.  Here at FINIS Australia we think saying it's a seriously smart swim tracker is getting closer.  The Apple Watch became water resistant in 2016 and OS upgrades delivered some next level metrics like set detection. 

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This watch knows when you rest at the pool's edge and then uses that information to divide the workout into sets of laps, showing you how far and long you swam in each, what stroke and your rest time.  All this from a gadget that was built as a watch!

Apple certainly love data.  In order to develop the algorithms to process the stroke of swimmers of all skill levels, they gathered data from more than 700 swimmers across more than 1,500 swim sessions. But wait, then they did more!  Apple also gathered data from people swimming in one spot while wearing a mask connected to monitoring equipment through the ceiling to capture everything at play in the session. That's a ship load of data and we at FINIS salute those who turns stats into better swimming.  There is a very good reason it took so much data to fine tune the Apple algorithm.  

Swimmers will be well aware skill level affects form but getting software to understand that is a tricky business. “If you are Michael Phelps, you’re distinctively making very good differences between the four strokes— backstroke, freestyle, breaststroke, and butterfly,” says Jay Blahnik, one of Apple's tech fitness and health gurus. “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.”

So for the tech stuff... a gyroscope and accelerometer track the motion of your strokes.  However, when you swim in open water, the algorithm supersizes it's combo by adding another sensor to the mix: the GPS chip.  When you take a dip in the ocean, river or lake, your watch uses GPS to determine how fast and how far you travel.

The development team did hit a small snag - GPS signals don’t travel through H2O. The good news is swimmers are likely to do freestyle in open water which means your arms regularly break the surface. The watch will set the GPS chip in acquisition mode for the whole swim. It goes hunting for a signal every time your hand comes up out of the water. “We’re trying to catch it every single time,” says Ron Huang, Apple’s director of engineering for location and motion services.

To recap, the accelerometer measures motion, and the gyroscope spots how many degrees the watch is rotating per second. Together, all that magic data helps Apple spot stroke type. Skilled swimmers do flip turns when they get to the end of the pool but no matter how you turn, the watch still needs to be able to detect it.  This is where the gyro earns it's keep. It measures rotation in three separate planes in space: around x, y, and, z axes. The x-axis goes horizontally across the display; the y-axis, vertically; and the z shoots straight out from the screen. Apple's software interprets the data from the gyro to figure out when you’ve reached a wall and turned 180-degrees in a different direction.

Before starting a swim, the watch will also prompt you for the pool length. By learning how many strokes it typically takes you to complete a lap, Apple knows when you are likely to make a turn.  If you normally need about 25 strokes, but later in the workout you take just 10 your watch will deduce that’s probably due to you taking a breather on the lane-dividing buoys. Every swimmer knows a strategic break works wonders, full credit to any watch that gets this too.

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