University of Illinois researchers figure that the same techniques that can be used for tracking missiles and space junk can make wearable fitness devices perform better.

They envisioned commercializing their motion-recognition technology for defense companies until Mr. Tilton was at a developers conference last year. A speaker was trying to demonstrate a next-generation wearable fitness tracker that automatically would recognize different physical activities. But the technology wasn't working very well because it was slow to recognize the different actions, Mr. Tilton said. "I figured, 'I can do better than that,' " he said. "We had spent years working on the technology. We had a prototype in about a month."

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