The Apple watch probably won’t capture the real-time heart rate data fitness geeks crave

Apple’s new watch is a second screen, a fashion piece, and a fitness tracker all in one, but it may not offer the kind of revolutionary fitness data that hardcore athletes are hankering for.

Its wearers will have the ability to track their heart rate (either for fitness and health reasons), which other wrist devices have so far failed to do in a way that’s design-friendly and affordable. Apple CEO Tim Cook said today at Apple’s launch event that the Apple Watch has a custom sensor (the four rings on the back of the watch) that “measures intensity by tracking your heart rate” by combining infrared sensors, visible-light LEDs, and photosensors. Apple says these measures will combine with an accelerometer and GPS to give a picture of pulse and activity.

The wording is somewhat vague, and it’s unclear whether the device aims to provide real-time heart rate data, or simply an estimate over the course of a workout or day to indicate the wearer’s intensity of activity or fitness level.

Measuring real-time heart rate data with any accuracy would be a big leap over popular current fitness trackers. Apple’s design offers more than the typical number of wrist-based sensors, but judging by Cook’s presentation, in total that may be closer to what’s already on the market, which uses optical sensing. By contrast, the tools doctors use measure electrical pulses.

Optical sensors are less accurate at measuring a person’s heart rate because they require illuminating the blood vessels on a person’s wrist with an LED in order to track blood flow. The measurement’s accuracy declines when a person is in motion, and is generally less precise because blood flow slows by the time it reaches your wrist. Monitors with chest straps that measure blood flow in motion tend to be more accurate.

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