This points to a potential answer to a long standing question I've had about why some hairs stop growing at certain lengths. If the force is being generated by cellular migration then control over when to stop growing can be mediated by a signal that tells the cells to stop migrating, and that could be based on time or vibration amplitude or something else that correlates with hair length. For hair that grows continually you just ... never turn off cell migration.
I don't believe any hair on the human body truly grows continuously. Even head hair has a lifespan of ~7 years and whatever you can grow in that time is the max. I was a big metalhead in high school and grew my hair out. Indeed, after about 7-8 years it stopped getting longer, right at about waist level, and was stuck there until I finally got sick of it and cut it off.
> why eyebrows stop growing and NOBODY ever thinks about it
If you clip your eyebrows, will they grow back to their original length? Or is there a process that generates an eyebrow hair and then stops after a pre-determined length of time (with periodic shedding)?
Every hair grows to a maximal length before stopping or falling out. It varies wildly based on genetics, location on the body, and body chemistry.
Animals evolved specialized hairs for different uses. Protection, warmth, display, your armpit hairs wick sweat and keeps your skin from rubbing. It's beneficial to have a system that keeps the specialized hairs in their optimal(ish) configuration and to replace hairs as they become worn and damaged.
