vibrators could be dangerous if used improperly ...
I still have a vibrator that my dad used to provide a "shoulder and neck massage" to patrons of his barber shop ca. 1948.
Customers apparently passed on favorable comments, so a couple of chiropractors visited the shop to request that he get them one, which he did.
About a year later one of the chiros came back and complained that they'd bought a replacement, and it didn't "vibrate" like the old one - it just sort of "hummed."
Asking at the local barber supply house, we were informed that a "new Federal regulation" (ca. 1949 or 1950) had imposed a limit on the "level of vibration" permissible, and the previous product was no longer available.
A letter to the manufacturer, Wahl, confirmed the same story.
Apparently - to protect "delicate tissues" where they might be used - there still is a "Federal regulation" that prohibits anything that "vibrates," although you can still get a "hummer."
(The currently available ones are much less effective, and less helpful for deep/large muscle relief - IMO. Maybe people now don't have as many big muscles and only use them on very small ones?)