This symphony of cracks and clicks makes you feel as though you can reach any key and can work faster and more dexterously than you could’ve before. Will you sound better now that you have cracked your knuckles? Nope – you don’t play piano; you’re here to move it to the next room.
Question of the Day – What makes the sound when you crack your knuckles and does it help or hurt you?
A knuckle is the joint where your carpal meets your metacarpal. A joint is any hinged meeting of two bones, made up of a sac (bursa) that keeps everything together and a small space between the bones (synovial space). Fluid in the space between the bones (synovial fluid) keeps the friction low as the bones flex in the joint.
Cracking joints can be done many at once or one at a time. Big joints can be cracked just as smaller joints. Chiropractic practices make a living out of cracking joints. There is an immediate feedback in hearing the joint crack; some treatment must have been rendered.
No matter how or why the joints are cracked, the production of the noise is the same in each case. Manipulation of the joint stretches the joint, separating the two bones, and creating a larger space than normal.
In physics, pressure is related to volume, being inversely related. The synovial joint has a certain amount of fluid, enough to fill the normal space. By increasing the joint space volume, the fluid will be filling a larger space. The same amount of fluid in a larger space means that the fluid will be under lower pressure.
This decreased pressure in your closed joint will affect the dissolved gases in the synovial fluid. All fluids of the body, like blood or other extracellular fluid, even saliva, contains many molecules, including dissolved gases; CO2, N2, O2. Gases in solution are under pressure just like in their gaseous state. When the pressure decreases, the amount of gas that can stay in solution decreases (it becomes less soluble). When gas becomes insoluble, it comes out of solution and begins to form small bubbles (ebulism, a gas embolism). It is the formation of the bubble(s) that you hear.
Cavitation (formation of a cavity) in small joints wouldn't seem to be a high-energy event, and it isn’t, but it is enough to make the noise you hear. There is still some question as to how such a large noise can be made this way, but x-rays can show the presence of gas bubbles in the popped joint immediately after cracking.
After cracking your joint, the joint remains a little larger for a period of time, and slowly returns to its normal volume. During the time period that the joint is returning to the normal volume, pressure slowly increases. More pressure means more gas solubility, and the bubbles disappear. It takes a while, so you can’t crack that knuckle again for 15-20 minutes.
Is it bad for you to crack your joints? Does it do damage to your joints, either immediately or over time? There hasn’t been a lot of research done in this area, but what has been done shows that knuckle cracking does not lead to osteoarthritis.
A 2011 study is the most comprehensive done to date. These researchers looked at cracking and the frequency of cracking as well. No amount of cracking seemed to promoted arthritis development. A 1990 study stated that knuckle crackers were more likely to also have small amounts of hand inflammation and lower grip strength. However, this study could not conclude that knuckle cracking caused the inflammation or loss of grip strength.
Oxygen’s counterpart, carbon dioxide, is also present in the blood, on its way back to the lungs to be exhaled. Most of the CO2 is locked up as part of carbonic acid (H2CO3) or its conjugate base, bicarbonate (HCO3), and this helps to maintain the pH balance of your blood. Yet there is a little free CO2 dissolved in your blood as well.
The major dissolved gas in your blood is N2, nitrogen gas. Remember that air 80% nitrogen. This also happens to be the most soluble gas in your blood, so more of your body’s allotment of nitrogen is carried this way; less need for carrier molecules like hemoglobin or bicarbonate.
This is all well and good until the pressure on your body changes, like when you go scuba diving. Water weighs much more than air, so for every 10 m (33 ft) you descend in the water, the pressure on your body doubles. With more pressure, more gas will be soluble in the blood. This is the opposite reaction from when you stretch your joints while popping your knuckles.
The increased gas volume dissolved in your blood is no problem as long as you allow it to dissipate slowly. But if you have been at depth for some time, and then you ascend too quickly, your body doesn’t have time to adjust to the change in pressure.
The return to normal pressure means less gas will be soluble in your blood. Where is all the gas you added to your blood by diving deep going to go? It’s going to come out of solution and form bubbles. This is decompression sickness, sometimes called the bends.
In the meantime, you are doubled over in pain (“the bends”). Pain is one thing, but if a bubble in your blood happens to get stuck somewhere, that’s called a gas embolus. Nothing downstream of the bubble is going to be getting oxygenated blood, and that means it will die. If it is in heart vessel, that causes a heart attack, if it’s in your lungs capillary beds, that’s a pulmonary embolism, if it is in your brain, that’s a stroke. Any of these can kill you.
The best way to treat the bends is to prevent them. You must ascend in stages, allowing time to adjust to the lower pressure at each depth. Your body will take the excess gas out of the blood if given time. When you learn to dive, much time is spent on the math involved in preventing decompression sickness; if you have been at such a depth for so long, you will need to ascend in X number of stages, with Y minutes at each stage depth.
If you don’t follow this, you’re in for a great deal of pain and a trip to a decompression chamber. In the chamber, they will pump in extra air to increase the pressure on your body, just like being at depth again. This will put the gas back into solution. Then they will release the pressure, a little at a time, allowing your body to take the excess gas out of your blood; the equivalent of a staged ascension for depth.
Even pressurized airliners have lower than normal air pressure (your ears pop), so no one is advised to fly after diving at depth for at least 24 hours. Dives that don’t require a staged ascent should still be completed at least 12 hours before flying. If you go diving in the morning, get on a flight immediately afterward, and then start popping your knuckles – could your hand explode?
Next week, another question in biology - can bacteria change the earth - the whole earth?
Blatteau, J., Brubakk, A., Gempp, E., Castagna, O., Risso, J., & Vallée, N. (2013). Sidenafil Pre-Treatment Promotes Decompression Sickness in Rats PLoS ONE, 8 (4) DOI: 10.1371/journal.pone.0060639
deWeber, K., Olszewski, M., & Ortolano, R. (2011). Knuckle Cracking and Hand Osteoarthritis The Journal of the American Board of Family Medicine, 24 (2), 169-174 DOI: 10.3122/jabfm.2011.02.100156