These questions are frequently asked in the clinic:
“Will my knuckles get bigger if I crack them?”
“My hip pops. Why?”
“Isn’t popping your hands a release of some gas?”
“It feels so good to pop my back and neck. Is that okay?”
“What is that sandpaper sound in my knee?”
There are several different joint sounds your body can make including cracking, popping, snapping, crunching, and clunking to name a few. These sounds are normal and generally do not require treatment, unless they are associated with pain. The cause of some of these joint sounds is unclear, but there are several different theories.
Cracking (i.e. cracking your knuckles) is thought to be a formation and collapse of a bubble. The bubble is generally thought to be a combination of dissolved gasses inside our joints, including carbon dioxide and nitrogen. However, a study recently published in PLUS ONE that used real time MRI imaging found that it is actually a vacuum-like cavity formation inside our synovial fluid (nutrient-rich fluid inside our joints) that causes the sound.  There is no evidence to say this makes your joints bigger. There is also no solid evidence to say this leads to arthritis.
Popping & Snapping
Popping and snapping (i.e. hip popping, shoulder snapping) can be a small misalignment and realignment between two bones in our bodies. This can occur from a positional change, or from a joint tracking malfunction. For example, “snapping scapula syndrome” is a common diagnosis to describe why your shoulder blade may make noise along your rib cage. The cause can be anything from weak or tight muscles, leading to poor shoulder blade (scapular) tracking, to a bony malformation acquired at birth (misalignment of the rib cage). Regarding the hip, snapping can be tendons rolling over our bones. For example, a tight iliotibial band (a fascial band that runs from your hip to your knee) could be sliding over the greater trochanter of your femur (the big bone that inserts into your hip socket). It also could be the hip flexor rolling over that bone (a deeper snapping sensation).
Crepitus is the name for the sandpaper sound your knees may sometimes make. It is caused by friction between our bones and cartilage (the stuff on the undersurface of your knee cap and between the other knee bones). With normal wear and tear over the years, the sound can increase. Good news - it doesn’t necessarily mean you’re going to need a knee replacement! It’s just a little annoyance, if anything. A colleague once described it similar to a light shaving of the surface of an apple. Apple is still good, right? Right. Our knees sometimes “creak” as well. This is the same concept of pressure on the cartilage sliding on a surface.
Clicking (i.e. hip and shoulder clicking) can sometimes be confused with snapping (the tendon rolling over a bone concept). It can also be bones hitting bones as they move against one another. In more involved cases, it can involve something called the labrum (a cartilage that lines the shoulder joint/hip joint). When the labrum is torn, it can catch inside the joint, producing a click. If you have a torn labrum that clicks, you generally will remember an injury with a lot of pain. Clicking without an injury or pain does not mean a labral tear.
The Bottom Line:
Something to emphasize is that these sounds do not necessarily mean you are damaging your joints. HOWEVER, if you are lifting at the gym, and you get a continuous, painful click in your shoulder, that is different, then consider talking to a PT or an MD. If you experience a pop with significant pain, swelling, and/or bruising, then you should consider getting immediate treatment. For example, your knee pops during soccer when you hyperextend and twist it, then it swells up and you want to cry. Uh oh! ACL tear, maybe? Or, you are playing tennis and your ankle pops. It feels like somebody kicked the back of your Achilles AND you can’t do a heel raise! Get that checked out.
Body Sounds Are… NORMAL
In summary, body sounds are normal. So, embrace your own body’s lovely joint sounds! We all have our own, fun, little, sometimes weird joint sounds. Just remember to consider your neighbor when you crack your knuckles in class. They can sometimes give people the heebie-jeebies, man!
Talk to me or my colleagues for more fun facts or if you have any questions or concerns.
 Kawchuck, Gregory N., Jerome Fryer, Jacob L. Jaremko, Zeng Hongbo, Lindsay Rowe, and Richard Thompson. "Real-Time Visualization of Joint Cavitation." PLOS ONE:. N.p., n.d. Web. 15 Feb. 2016.