What is the cracking or popping noise during a manipulation?
Synovial fluid present in your joints acts as a lubricant. The fluid contains the gases oxygen, nitrogen, and carbon dioxide. When you pop or crack a joint, you stretch the joint capsule. Gas is rapidly released, which forms an audible sound. Although this is not necessary for manipulation to be effective, it is simply the sound of the joint suddenly moving or releasing from its restricted state.
