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The smartwatch arena may soon have yet another tech titan challenging the rest: A report from Forbes says Microsoft is preparing to jump into the digital fitness world in a big way, perhaps as early as this summer.

Citing unnamed sources, Forbes says it has learned Microsoft is developing a heart rate-monitoring device worn on the wrist — but rather than having the screen sitting on the outside of your wrist, it would be slung on the inside.

Why? Not just to be different, or more convenient, though a watch worn that way can be easier to consult while running or driving. It may have something to do with the technology used to detect your heartbeat.

From left, fitness trackers Fitbit Force, Jawbone Up, Fitbug Orb and the Nike FuelBand SERichard Drew / AP file

Forbes' source claims that some "expertise" from the Kinect, Microsoft's motion-detecting add-ons for Xbox video game consoles, went into the watch — perhaps some kind of high-powered infrared sensor that detects changes in your wrist's blood vessels. And since your blood vessels are closer to the inward side of your wrist than the outside, it would make sense to put the sensor there.

That's all speculation, of course, and to round out the rumor is the idea that the smartwatch will be cross-platform, working with iOS, Android and Windows Phone devices. That makes sense: Not only has Microsoft has been on something of a compatibility spree lately, expanding the capabilities of its OneDrive and Office service on other platforms, but limiting a device to only work with Windows or Windows Phone devices would shut out about 96 percent of the mobile market.

NBC News has contacted Microsoft for comment and will update this post if we hear back.