Why do our knuckles make that great sound when we crack them?
If you've ever laced your fingers together, turned your palms away from you and bent your fingers back, you know what knuckle popping sounds like. Joints produce that grand CRACK when bubbles burst in the fluid surrounding the joint.
Joints are the meeting points of 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, clear liquid.
When you stretch or bend your finger to pop the knuckle, you are causing the bones of the joint to pull apart. As they do, 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.
So as the pressure of the synovial fluid drops, gases 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 25-30 minutes for the gas to redissolve into the joint fluid. During this period of time, your knuckles will not crack. Once the gas is redissolved, cavitation is once again possible, and you can start popping your knuckles again.
As for the harms associated with this habit, only one in-depth study regarding the possible detriments of knuckle popping has been published. This study, done by Raymond Brodeur and published in the Journal of Manipulative and Physiological Therapeutics, examined 300 knuckle crackers for evidence of joint damage.
The results revealed no connection between knuckle crackin' and arthritis.
In fact, there is evidence of increased mobility in joints right after popping. When joints are manipulated, the Golgi tendon organs (a set of nerve endings involved in humans' motion sense) are stimulated and the muscles surrounding the joint are relaxed.
This is part of the reason why people can feel "loose", orgasmic and invigorated after leaving the chiropractor's office, where cavitation is induced as part of the treatment. Backs, knees, elbows and all other movable joints are subject to the same kind manipulation as knuckles are.
Ahhhh, I love a good crack.