What causes joints to crack?
Your joints can make a whole variety of cracking, grinding, snapping and popping sounds and the chances are, that you know someone who just loves to crack their knuckles. The joints that 'crack' are the knuckles, knees, ankles, back, and neck, and this is the usual reason why they 'sound off'.
Joints are the meeting points of two separate bones, which are held in place by connective tissues and ligaments. All of the joints in our bodies are surrounded by a thick, clear liquid called Synovial Fluid. This fluid contains the gasses oxygen, nitrogen and carbon dioxide. When you stretch or bend your finger to pop the knuckle, you're causing the bones of the joint to pull apart. As they do this, the connective tissue capsule that surrounds the joint is stretched and its volume is increased. If you are at all interested in chemistry, you will know that, with an increase in volume comes a corresponding decrease in pressure.
As the pressure of the synovial fluid drops, gases dissolved in the fluid become less soluble, and form bubbles through a process called cavitation. If the joint is stretched far enough, the pressure in the capsule drops so low that these bubbles burst, producing the pop that we associate with knuckle cracking. In order to crack the same knuckle again, you have to wait until the gases are redissolved into the synovial fluid and cavitation is once again possible. Commonly, when a joint moves, in the knee or ankle, the position of the tendon changes and moves slightly out of place. You may hear a snapping sound as the tendon returns to its original position. In addition, your ligaments may tighten as you move your joints and can make a loud cracking sound. This is different from the grinding noise Arthritic joints can make as these sounds are caused by the loss of smooth cartilage and the roughness of the joint surface.
Is joint cracking harmful?
Research shows that simple clicking in the joints is extremely common and unlikely to be related to any kind of disease. Some people have slightly lax joints and when this is the case the joint may click. So provided that when the joint clicks it isn't associated with pain, swelling, redness or a reduction in the joint's movements, then all is probably well. Over-training can sometimes cause soft tissue inflammations around your joints, and minor trauma such as a sprain may cause the joint to start clicking because inflammation and damage has disrupted the stability and organisation of the supporting muscles and tendons. Once this has healed however, the clicking disappears. In fact, it is often the case that a person won't even be aware that an injury has happened. In terms of knuckle cracking, some studies show that knuckle cracking does not actually cause any serious harm. Other studies show that habitual knuckle cracking can, over time, create damage to the soft tissue of the joint and may possibly lead to a decrease in grip strength.
On the positive side, there is evidence of increased mobility in joints straight after popping. This is because, as the joints are manipulated, the Golgi Tendon Organs (a set of nerve endings involved in our Motion Sense) are stimulated, and the muscles surrounding the joint are consequently relaxed. So if you go to a Chiropractor or have a treatment where Cavitation is induced, perhaps with Osteopathy, Sports or other types of massage, you will feel loose and invigorated. Backs, knees, elbows and all other movable joints are subject to the same kind of manipulation as knuckles are.