What type of sensors are these?
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.