Knuckle cracking

patterson at frodo.mgh.harvard.edu patterson at frodo.mgh.harvard.edu
Fri Oct 14 11:52:27 EST 1994


In article <37lrue$20h at nomad.urich.edu>, radice at urvax.urich.edu wrote:

> During a lecture about joint anatomy, a student asked what makes the
sound when 
> one "cracks" ones knuckles.  I wasn't sure: I seem to remember competing
> theories about ligaments snapping like rubber bands, or gasses expanding or
> collapsing. The two main questions seem to be, what causes the sound, and why
> is there a refractory period before you can make the sound again.  I would go
> to the library for this, but I really have no idea where to begin
looking.  Any
> body have a good answer?

This question is addressed in "The Straight Dope", by Cecil Adams.  He says
that finger cracking is due to expansion of a cavity in the knuckle.  The
cavity is full of fluid, and when the "negative" pressure is great enough,
a bubble of
gas is formed.  When the bubble collapses, a pop is heard.  The bubble doesn't
completely dissappear, so trying to crck again results in slow,
non-catastrophici bubble expansion and contraction.  Hence, the refractory
period.  Pops in other areas of the body (e.g. your back) are perhaps due
to gas bubbles or perhaps due to the rubber band theory you mentioned.  I
am guessing
that your library doesn't have The Straight Dope, but I have a better idea.  Try
the alt.fan.cecil_adams newsgroup!  Send your question there, and see if
someone will quote Cecil's answer.

-- 
Garth I. Patterson
Dept. Molecular Biology
Massachusetts General Hospital



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