Why do joints bend?

At first glance, the joints sound fresh. But from a physical point of view, the movement of joints is nothing more than sliding (such as the initial bending of the knee), rolling, grinding (rotation) and so on. These have no mechanical principle, and naturally sound will be generated due to friction and vibration, just because the joint cavity is tightly closed, the joint synovial fluid is effective and unobstructed, and the sound is too small to be heard. When the joint "moves" for the first time, the force of tendon (such as tension) will deform the joint instantly, which can be increased to 7% at most, and the deformation will produce negative pressure in the joint cavity. Although this negative pressure is small, it is enough to depress (inhale) the joint capsule, and partially vaporize the gases such as nitrogen, oxygen and carbon dioxide dissolved in synovial fluid into bubbles. When the action is about to be completed, the bubbles and synovial fluid are compressed, that is, the sunken joint capsule is bounced back like champagne, and the gas is dissolved back into the liquid. Because it takes time for the gas to dissolve again, the knuckle sound can be repeated, but it cannot be repeated. Of course, in addition to this mechanism, the knee joint, ankle joint or back make noise, or during the joint movement, the taut tendon may "walk" slightly and then bounce back to its original position; Or there is a big friction with the articular process, making a noise. If it is not accompanied by redness, swelling, heat and pain, it should generally be fine.