With questions about popping knuckles, I went to Marie, a young lady in my circle who I knew was an expert on the topic.
I asked her to demonstrate the art. She promptly put her hands together as if in prayer, then forced her fingers backward one way and then the other. A cluster of rapid-fire pops shot out as she hyperflexed and hyperextended fingers with ease and a smile. For as common as knuckle popping is, there's a still a lot we don't know.
Research speaks of articular separation (the space produced in the joint when bent), gases dissolved in synovial fluid (bubbles) and joint cavitation (a small vacuum formed in the joint space by the bubbles). But then things get blurry. Do the bubbles pop, or do ligaments snap, or do vacuums slip from one space to another, or do joint adhesions break? All of the above are possibilities, but we just don't know.
In the 1970s, a machine was created to pop knuckles so it could be studied more thoroughly. Of 17 subjects tested, only five produced cracks. Seven did not crack, and five wouldn't relax enough for the test to continue. Of the five crackers, a crescent-shaped space (cavitation) was seen between the finger joints just after the pop (0.98 mm of joint separation before cracking, 2.5 mm after cracking).
Seven brave test subjects were pricked with a needle in their finger joints in order to collect synovial fluid. From that, we learned that in the volume of liquid collected, the average gas content is 15 percent – with over 80 percent as carbon dioxide. That's what forms the bubble.
Not to be outdone by these research participants, I asked Marie if she could crack my knuckles. She promptly took my hand, bent my finger forward on itself and pressed. Hard. I think I heard a pop, but it was somewhat masked by my yelp of pain. I guess I'm not a knuckle-cracker.
Other joints pop similarly, which makes our quest for knuckle-popping knowledge even more confusing. When our lower back pops from spinal manipulation techniques (think chiropractors) the zygapophyseal joint of our lumbar spine gapes (men wider than women). And we know that spinal manipulation of the neck can be dangerous, even in the hands of professionals. In this review of effects, more than 200 patients were suspected to have been seriously harmed. Mild adverse effects occurred in 30 to 61 percent of the patients.
But is knuckle popping dangerous? Multiple studies have found no connection between this and arthritis. The worst outcome may be some mild looseness of the joints, but again … we're not sure. Studies suggest that once fingers are popped, they cannot pop again for approximately 15 to 20 minutes. The theory there is that the gas “bubbles” must diffuse back into the synovial fluid before the next pop can occur.
So I tested this out with Marie. Asked her to try to pop her knuckles again a couple minutes after the first round of cracks. She put her hands together and her fingers rattled off a new bundle of pops, much like the first. I guess there's still a lot to be learned.
Can you stand watching 4 minutes of knuckle popping?