HOW COME?: When knuckles crack and joints squeak

How come your fingers pop when you apply pressure to them, like when you crack your hands before taking a test or starting a competition? asks reader James Zipf.

It can feel like the hand's version of a satisfying yawn. Human beings have probably been lacing their fingers, stretching out their arms and cracking their knuckles for thousands of years. But it wasn't until the 1970s that scientists began to figure out how the popping worked.

Take a look at the top of your hand: The knuckles are the joints linking your hand bones (the metacarpals) to your finger bones (the phalanges). The knuckles allow us to grip objects between our fingers, from pencils to pliers to pennies, using a system of ligaments and tendons.

Inside the knuckles, just as in knee, elbow and shoulder joints, there are spaces between bones. In these spaces, providing helpful cushioning and reducing friction, lurks a thick liquid called synovial fluid. Synovial fluid, which is the consistency of raw egg, contains dissolved gases like carbon dioxide and oxygen. This slippery, gassy fluid is trapped in a fibrous capsule surrounding the joint.

But when you pull your fingers back, you widen the space between the joints, stretching out the fluid-filled capsule. The result: With its holding tank expanding like a balloon, the synovial fluid is suddenly under a lot less pressure.

Stretching and pulling on your knuckles is rather like shaking a can of soda pop. As the synovial fluid's pressure swiftly drops, its trapped gases begin forming scattered pockets of vapor. Presto: Downright bubbly joint fluid. As you stretch the joints to the max, the pressure drops so low that the bubbles burst. Mini shock waves race through the synovial fluid. And we hear a crack or pop - a tiny sonic boom, finger-style.

But even as the bubbles burst with an audible pop, the gas doesn't immediately redissolve in the joint fluid. Small, lingering bubbles keep the joint capsule expanded. In fact, knuckle X-rays show that it takes up to 30 minutes for the tiniest bubbles to completely disappear. As the gas gradually dissolves, the capsule shrinks back to its normal, compact size.

And now you know why it's nearly impossible to crack your knuckles twice in a row. The synovial fluid capsule can't form big, pop-tastic bubbles if it hasn't first shrunk back to normal. In fact, impatiently pulling on your fingers while the capsule is shrinking back only makes the dissolving bubbles re-expand a bit. But let the gas fully dissolve, and the stage is set for another loud pop.

Knuckle-cracking can be annoying to those around us, but scientists say it doesn't cause arthritis. Arthritic pain, stiffness and swelling in joints can be caused by an overactive immune system (rheumatoid arthritis) or by cartilage wearing away over time (osteoarthritis). Finger joints may feel temporarily looser after knuckle-cracking, just as other joints feel looser after a good stretch. However, one study found that too much knuckle-cracking may injure the soft tissue around the joint, causing swelling and reduced grip strength.