I'd be interested to see a long term study of fix gear riding vs with a freewheel. (Freewheel in the sense of a mechanism that can "coast", not freewheel vs cassette. So, in this sense, fix gear vs "freewheels", freehubs, IGHs, etc,)
I'd offer myself as a subject, I've ridden each roughly equally over 45 years. Yes, I certainly have suffered the bone loss typical of cyclists (get told every X-ray) but I also have always felt that riding fix gear did far more for me physically than riding freewheeling bikes as a person in the everyday world. I see that whenever I do yard work, handle power tools or do a myriad other physical tasks. When I return to weightlifting, I start with more weight if I've been riding fixed. I look better in the mirror. Shirts fit better. No, none of these perks come close to what I achieve doing actual weight lifting. But I can have these small perks simply doing the same rides I'd do with gears.