What causes the noise heard when people crack their knuckles

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The synovial fluid needs to expand when the joints are moved, to allow this movement. However, it can not do so until extra space has been created within the joint. It's thought that the cracking noise that people make with their knuckles is caused by the pressurised gas coming out of solution, increasing the volume, and so the mobility of the joint.

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X-rays of knuckles taken just after they have been cracked have revealed tell tale gas bubbles inside the joint.

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Q.� Why can't the same joint be cracked over and over

A.� Because you have to wait for the gases to become dissolved back into the synovial fluid to re-create the cracking sound.

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Q.� How can the release of such a small amount of gas create such a loud crack

A.� The scientists don't understand this one completely. The amount of energy created by the release of the gases is indeed very small, not really enough to warrant such a relatively loud crack.

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Q.� Can any joint be cracked

A.� The ones that are 'crackable' are those known as the diarthrodial joints: joints that contact each other at their cartilage surfaces, the most common type of joint.

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Beyond this, the easiest to crack are the finger joints.

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Q.� Is cracking knuckles dangerous

A.� Not many medical studies have been devoted to the side effects of this some times annoying habit. Where studies have been carried out the results vary, with some indicating onset of arthritis associated with knuckle cracking while other show no adverse effects

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Q.� What are the cracking noises my knees make when I stand up sometimes

A.� The tendons and ligaments near your knee joints, which snap out and then back into position as you move your knee, and, therefore, the knee joint, which the tendon moves, cause these.

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Do you have a question about why your body does something Click here to ask.

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by Lisa Cardy