Eli5: Why do our joints get stiff and need to be ‘cracked’ if they’re inactive for a few hours? What’s happening inside?

Question

Eli5: Why do our joints get stiff and need to be ‘cracked’ if they’re inactive for a few hours? What’s happening inside?

In: Biology

Answer 1

There’s a liquid in your joints called synovial fluid. It acts as a lubricant and protectant to the joints, allowing them to move smoothly.

When you let your joints sit for a while, gas bubbles can enter the fluid. These bubbles are what cause the popping noise: you’re literally “popping” these bubbles when you crack your knuckles/knees/back.

Answer 2

I can crack my knuckles none stop, I wonder what that means?