I saw this sensors in an YouTube video where these things were glued to Glenn Maxwell's dress and his whole movement was monitored for the making of a cricket game.
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I don't think those are sensors. I think those balls, against a black background, are a constellation that can be dealt with efficiently using computer vision techniques from normal camera output. Algorithms that know something about how they are distributed on a moving body can infer the movement of that body through space mapped onto a 3-D model with hinges and joints. The technology as a whole is called "motion capture" I believe.
Maybe they are also highly reflective to IR or something, but I seriously doubt they are "active" in any way.