I Knew Our Parents Were Lying! (page 8)
Cracking your knuckles will give you arthritis
Another incredibly well-known “fact” that parents share with their children, the sound of the knuckles cracking is caused by bubbles of gas popping inside the joint. But through all the studies which have been done on knuckle-cracking, none of them have found an increased tendency toward arthritis. Other issues that have been identified as a result of long-term cracking include ligament damage and reduced grip strength. These are significant enough by themselves to deter most people from this habit, but arthritis is not a legitimate argument to make.