Science

What happens when we crack our knuckles?

We're all familiar with the loud pops that come with stretching out our knuckles to their limits. For some of us, it's a comforting noise that we associate with limbering up. For others, it's a spine-tingling annoyance. Either way, you've probably wondered, what happens when we crack our knuckles? How can a simple stretch make so much noise?

Joint construction

Woman holding hands together showing bent knuckles
Credit: AndreyPopov/ iStock

Almost any joint in the human body can make a cracking sound if it gets stressed in the correct way, but some do this more easily than others. Your knuckles are one of the easiest types of joints to crack since they're diarthrodial joints.

Diarthrodial joints are a connection of the ends of two pieces of bone at their cartilage surface. These cartilage structures on the bone are encased in a joint capsule. In addition to keeping the two bones in contact, the joint capsule keeps the synovial fluid that facilitates free movement of the joints in place. This synovial fluid also serves as a delivery medium for the nutrients that feed the cells that make up the cartilage and keep it healthy.

Joint stress leads to expansion

So how does this structure lead to those loud popping noises when joints are cracked? The synovial fluid contains nitrogen, oxygen, carbon dioxide, and many more gases that have been dissolved as they were used to power the cells. When you stretch out your knuckles, the joint capsule is squeezed. As pressure is applied to it, gas is released to make more room.

As the gas is rapidly expelled from the joint capsule, it forms small bubbles. The cracking that you hear is the sound of these bubbles escaping and popping. This process depletes the amount of gasses that are in the joint capsule. If you want to crack your knuckles again, you have to wait for the gas to replenish itself. For most people, this process takes about twenty minutes, and it’s the reason why you can’t just pop your knuckles over and over again.

Is popping your knuckles bad for you?

Person cracking knuckles
Credit: Andrey_Popov/ Shutterstock

Because of the loud disruption that popping your knuckles creates, many people assume that doing it often, or over the course of a lifetime, will have negative consequences. However, the scientific community has yet to reach a consensus on the long-term effects of habitual knuckle cracking.

Some studies have found a link between decreased grip strength and regular knuckle cracking, while most doctors agree that if cracking your knuckles causes you pain, you should visit a health professional. However, one of the most comprehensive studies was performed by a man named Donald Unger, who methodically cracked the knuckles on his left hand and not his right hand over the course of 60 years. Near the end of his life he did experience some problems with arthritis, but no more in his left hand than in his right.

What about the other joints?

You may be thinking about other joints in your body that make a lot of noise. Do these work the same way? Many of them do, as a lot of the important joints in our bodies are diarthrodial just like our knuckles. However, some of our larger joints have different behaviors that make noise as well.

For example, with larger joints such as our ankles or knees, some of the sounds are actually the physical movements of components such as tendons, joints, and ligaments. When you make a sudden movement or put a lot of strain on a joint, it can cause the tendons to move slightly out of place. When it moves back, a snapping sound can be heard.

Another cause of loud joints is arthritis. As arthritis begins to affect the human body, joints lose the smooth cartilage that allows us to move freely. The roughness of the joint surface causes various cracks and other sounds that are clearly audible. So if you like cracking your knuckles, then crack away, and if not — well, there’s no downside to abstaining from it either.