What makes your knuckles pop?

Learn more about the anatomy (and chemistry) behind that popping sound your knuckles make when you crack them in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:00):
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(00:23):
from how Stuff works dot com where smart Happens. I
am Marshall Brain with today's question, what makes your knuckles pop?
If you've ever laced your fingers together, turns your palms
away from you and bent your fingers back, you know

(00:44):
what knuckle popping sounds like. Joints produced that crack when
bubbles burst in the fluids surrounding the joint. Joints are
a meeting point between two separate bones, held together and
in place by connective tissues and ligaments. All of the
joints in our bodies are surrounded by synovial fluid, a thick,

(01:05):
clear liquid. When you stretch or bend your finger to
pop the knuckle, you're causing the bones of the joint
to pull apart. As they do this, the connective tissue
capsule that surrounds the joint is stretched. By stretching this capsule,
you increase its volume, and, as we know from chemistry class,
with an increase in volume comes a decrease in pressure.

(01:29):
So as the pressure of the synovial fluid drops, gas
is dissolved in the fluid become less soluble, forming bubbles
through a process called cavitation. When the joint is stretched
far enough, the pressure in the capsule drops so low
that these bubbles burst, producing the pop that we associate
with knuckle cracking. It takes about five to thirty minutes

(01:51):
for the gas to redissolve into the joint fluid. During
this period of time, your knuckles won't crack again. Once
the gas is dissolved, cavitation is once again possible, and
you can start popping your knuckles again. As for the
harms associated with this habit, according to Anatomy and Physiology
Instructors Cooperative, only one in depth study regarding the possible

(02:16):
detriments of knuckle popping has been published. This study, done
by Raymond Broder and published in the Journal of Manipulative
and Physiological Therapeutics, examine three hundred knuckle crackers for evidence
of joint damage. The results revealed no apparent connection between
joint cracking and arthritis. However, habitual knuckle poppers did show

(02:40):
signs of other types of damage, including soft tissue damage
to the joint capsule and a decrease in grip strength.
This damage is most likely a result of the rapid,
repeated stretching of the ligaments around the joint. On the
positive side, there's evidence of increased mobility and joints right
after popping. When joints are manipulated, the gold g tendon organs,

(03:05):
a set of nerve endings involved in humans motions, sense
are stimulated and the muscles surrounding the join to relax.
This is part of the reason why people can feel
loose and invigorated after leaving a chiropractor's office where a
cavitation is induced as part of the treatment. Backs, knees, elbows,
and all other movable joints are subject to the same

(03:27):
kind of manipulation as knuckles are. Do you have any
ideas or suggestions for this podcast? If so, please send
me an email at podcast at how stuff Works dot com.
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