That's a loaded question, not all adjusting techniques chiropractors use produce the popping noise that we associate with "cracking your knuckles." In fact, some chiropractic adjusting techniques use little force and make no sound. Others may use adjusting tables with drop sections which make so much noise that any sound from the patient's spine can't be heard. Many chiropractors may and do use techniques that create the popping sound during a spinal 'release'.

The noise, what causes that? A few years ago the mystery was most likely solved. A British research team took X-ray movies of a person "popping" his knuckles and found that gas (80% carbon dioxide) rushes in to fill a partial vacuum created when the joint surfaces are slightly separated. It is this displacement of joint fluid which some believe to be the cause of the noise during an adjustment. It is NOT the sound of a joint "going back into place".

(References: Kirkaldy-Willis, WH. (ed.) Managing low-back pain (2nd edition), Baltimore and London; Williams and Wilkins, 1988. Imrie, D. and Barbuto, L., The back power approach. Toronto: Stoddard Publishing, 1988.)