What Causes our Joints to “Crack?”

According to Harvard Medical School, “the ‘pop’ of a cracked knuckle is caused by bubbles bursting in the synovial fluid — the fluid that helps lubricate joints. The bubbles pop when you pull the bones apart, either by stretching the fingers or bending them backward, creating negative pressure. The reason you can’t crack the same knuckle or joint twice right away is that it takes some time for the gas bubbles to accumulate again in the joint.”

Is Knuckle Cracking Harmful?

It is common advice not to crack one’s knuckles or other joints for fear of causing arthritis. (Related IFOD: Is Running Bad For Your Knees?)

After receiving advice not to crack his knuckles as a youth, Dr. Donald Unger, a physician, tested the hypothesis of whether cracking knuckles would cause damage to his joints. From his research paper: “For 50 years, the author cracked the knuckles of his left hand at least twice a day, leaving those on the right as a control. Thus, the knuckles on the left were cracked at least 36,500 times, while those on the right cracked rarely and spontaneously.” What did he find out? “There was no arthritis in either hand, and no apparent differences between the two hands.” He concluded that “there is no apparent relationship between knuckle cracking and the subsequent development of arthritis of the fingers.” As fun as Dr. Unger’s study is, it is based on small sample size. Other studies, such as this one, have also found that cracking joints does not cause arthritis and it is accepted by the meidcal community that cracking joints does not lead to arthritis.

Note, however, that this study from 1990 did find reduced grip strength in knuckle crackers and thus recommended against knuckle cracking. Interestingly, the study also found that habitual knuckle crackers “were more commonly manual labourers with higher incomes who more frequently smoked, drank alcohol, and bit their nails.”