As I’ve aged, evidence of my lost cat-like stealth comes in the form of various snapping, crackling and popping noises I make when I walk. They’re freakish sounds, though I have no pain associated with it. In fact, it actually feels good when I stretch at night.
The most common popping noises people have are associated with knuckle cracking. (… which I don’t do.) Knuckle joints look like the diagram to the right: two bones contact at their cartilage. Cartilage is surrounded by synovial fluid — a soluble lubricant. All of this is surrounded by a joint capsule.
Raymond Brodeur describes the physical action of this kind of popping:
“When small forces are applied to the joint, one factor that limits the motion is the volume of the joint […] set by the amount of synovial fluid contained in the joint. The synovial fluid cannot expand unless the pressure inside the capsule drops to a point at which the dissolved gases [Nitrogen, CO2] can escape the solution; when the gases come out of solution, they increase the volume and hence the mobility of the joint.
The cracking or popping sound is thought to be caused by the gases rapidly coming out of solution, allowing the capsule to stretch a little further. The stretching of the joint is soon thereafter limited by the length of the capsule. If you take an x-ray of the joint after cracking, you can see a gas bubble inside the joint. This gas increases the joint volume by 15 to 20 percent; it consists mostly (about 80 percent) of carbon dioxide. The joint cannot be cracked again until the gases have dissolved back into the synovial fluid[…]”
These noises are generally assumed to be harmless unless there is pain, swelling or loss of mobility. In one study of 300 people who had been cracking their knuckles for over 30 years, many with swollen ligaments and a profoundly weaker grip.
Another source of popping sounds is the movement of joints, tendons and ligaments. Tendons cross over a joint, but when joint moves, the tendon can sometimes snap, causing a noise. Adds Dr. Brodeur, “These noises are often heard in the knee and ankle joints when standing up from a seated position or when walking up or down the stairs.”
Ah ha! Sort of. This is what I hear as I go upstairs to bed each night. More recently, as I’m lying in bed at night, I have the urge to “stretch” which effectively causes a series of joyful crackling. Again, no pain or swelling or anything. It just sounds creepy.
A final source of noise is arthritis. The joint’s roughness, caused by deterioration of cartilage and bone spurs, can result in grinding or cracking noises when the joint is moved. I was surprised that there are over 100 different types of arthritis caused by a host of factors including auto-immune disorders.
-  “Scientific American’s Ask the Experts,” Raymond Brodeur, Ergonomics Research Labratory, Michigan State University.
-  Everyday Mysteries, Library of Congress
-  “About Joints,” UW Medicine Orthopaedics and Sports Medicine.
-  “Arthritic Conditions,” UW Medicine Orthopaedics and Sports Medicine.
-  “Knuckle Cracking,” Karl S. Kruszelnicki.
-  “Patient Guide to Joint Cracking & Popping,” Johns Hopkins Medicine.