Chandana Chandra explains: A click or a popping sound is often heard when a joint is moved or stretched. Actually, two separate and distinct sources are responsible for the familiar popping sound that our joints emit.

Joints are the meeting points of two separate bones, which are held together by connective tissues and ligaments called tendons. A thick clear lubricant called the synovial fluid cushions the joints. The fluid comprises mainly of carbon dioxide and nitrogen. When you stretch or pull your fingers to get the desired popping sound, you actually pull the bones apart. Thereby reducing the pressure on the synovial fluid. This reduced pressure causes ‘cavitation’ (withdrawal of carbon dioxide and water vapour from the fluid). The low pressure collapses instantly and the fluid crashing in from all sides makes the noise.

It takes about 15 to 20 minutes for the carbon dioxide to redissolve. The cracking sound can’t be reproduced unless and until the synovial fluid reabsorbs the gas. But on the other hand, when we do deep knee bends a snapping sound is heard.

That sound is produced when our tendons elastically snap into new positions as our joints move under stress. We don’t have to wait to reproduce this snapping sound, as the tendon contracts and relaxes with the movement of the joint.

The question was sent by Harsh Bhandari from Shibpur, Howrah