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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. Even if knuckle cracking doesn’t cause arthritis, there’s still good reason to let go of the habit. Chronic knuckle-cracking may lead to reduced grip strength. And there are at least two published reports of injuries suffered while people were trying to crack their knuckles.
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