Creak! Crack! A Story of Knoisy Knuckles

Last Updated (Wednesday, 28 October 2009 10:03) Written by Raymond Nakamura

One morning, my five-year-old daughter asked me why my body made so many cracking sounds, my feet in particular. Her joints did not make so much noise, though of course her voice more than made up for this silence.

I told her about the gases in the fluid in your joints making bubbles that popped and how it takes a while for the gas to accumulate, so you can't do it again right away. But she wanted to know why hers didn't crack and I didn't know. I explored this joint venture a little further.

Is It Wise to Crack?

Some treatments involve manipulating your joints, especially your spine. Usually the sound of the pop is used to indicate if it has been performed correctly. Yet one study indicated that the sound was not an indicator of effectiveness. The technique was not all it was cracked up to be.

Recently, a man received an Ig Nobel prize for his life long experiment on himself, cracking the knuckles of one hand and not the other for FIFTY YEARS. He claimed that he hadn't developed arthritis and didn't notice any difference between his hands.

Other studies also failed to find a link with arthritis, although some suggested that it can result in more swollen joints and less hand strength. Though I wonder if the tendency to crack your knuckles is independent of these other characteristics. I liked this part, "The chief morbid consequences of knuckle cracking would appear to be its annoying effect on the observer." If you do annoy others with your cracking and would like to stop, this link has some advice.

(Not intended to besmirch the reputation of spiders, bats, the full moon, assembled persons, or anyone living or undead)

It's a Gas

When the joint is pulled, this increases the volume and decreases the pressure, so that bubbles (carbon dioxide and water vapour) form, like when you undo the cap on a bottle of pop. The sound comes from the implosion of bubbles not an explosion. An expansion on the explanation suggests that the sound is actually two separate things happening: bubbles popping and the ligaments of the joints snapping back, at least in some cases. Another variation suggests that the bubbles get shifted between areas of high and low pressure. But still, it was all talk and no data. So I was still a bit skeptical of the bubble explanation.

Claws and Effect

I wondered if any other phenomena were related to knuckle cracking that might illustrate the point. I remembered snorkelling in Thailand and hearing pistol shrimp making cracking sounds under water. It sounded like rain. So I looked up whether anyone had studied what pistol shrimp do. I was surprised to see that the sound was attributed to collapsing bubbles rather than to the snapping of the claws. They had high speed footage indicating how the rapid movement of the claw produced bubbles and when they collapsed, the sound was heard.

Pump It Up

This phenomenon of bubbles being formed by lowering the pressure of a fluid is called cavitation. Apparently, it is a concern for submarine propellers and pumps, or at least for the people who deal with them. I recently had an unpleasant saga involving aging sump pumps, so I was curious about the connection. I found this article suggesting a link between cavitation, pumps, and knuckle cracking. 

So the gas explanation seems plausible enough, though I still haven't seen good data for it. Based on it, I can only speculate that my daughter's smaller joints are not able to make as much gas to be audible. And maybe my circulation is slowing down as I get older. Though I'm not sure that's really something I wanted to hear.