Why Knuckle Cracking Doesn't Cause Harm:

According to Sanjiv Naidu, Penn State professor of orthopedicts:
"Plain old knuckle cracking should not cause any damage. It does not strain the ligaments or the tissues, or overextend them enough to cause arthritis," Naidu said. "It also should not cause joint weakness, on a long term basis. Anatomically, physiologically and mechanically, there's no reason it should cause harm."
Naidu elaborates, "You literally have to disrupt the joint capsule through excessive force--like a ligament injury in a knee, or 'skier's thumb,' for example-to cause chronic, long-term damage." The forces generated by knuckle-cracking are relatively small in comparison, Naidu said.
(http://www.physorg.com/news64162917.html)