When the Chiropractor adjusts a joint he must first take up all the "slack" in the joint so as to control the movement perfectly. There the patient must contribute by relaxing completely to minimise any discomfort. The Chiropractor aims to induce a split-second stretch to the joint capsule and associated muscles and tissue fibres. By taking up the slack the joint is opened as far as it can voluntarily go, a point referred to by Chiropractors as the "elastic barrier or resistance". There is still ample room for the joint to move within its anatomical range, but if the movement is a slow stretch it triggers a natural resistance from the tissues of the joint.
The speed of the Chiropractic adjustment is such that joint tissue resistance is not triggered and the elastic barrier is overcome, often producing a joint "crack". The main effect of the rapid stretch is a neurological response, which creates a wave of relaxation within the joint tissues, thereby restoring mobility. In cases where ligaments have physically shortened, the joint will tend to stiffen up again, although usually not to the same extent as before the adjustment. In time, with repeated adjustment at regular intervals, short ligaments will lengthen again, restoring full function to the joint. It was once thought that the bones made the joint-crack as they "popped back into place".
Another theory was that the noise came from ligaments as they slapped tissues on being fully stretched. Researchers at Leeds University showed that the pop occurred within the fluids of the joint capsule. The sudden vacuum caused by stretching the joint brings gases out of the fluids in the form of tiny bubbles. As the vacuum increases within the joint fluids, the bubbles come together until they form one sizeable bubble. The next phase occurs as the bubble bursts with a "pop". All this happens painlessly in a fraction of a second. Although it is an indication that a joint has been adjusted, the "crack" is not essential to the success of the adjustment, as it is the rapid stretch and not the bursting bubble that induces tissue relaxation and increased mobility.
Indeed, Chiropractors have devised numerous methods over the years for restoring full mobility, not all of which require the mediation of the classic Chiropractic adjustment. There are instances when the adjustment is not called for, as Daniel Palmer's son, "B.J.", recognised when he wrote: "It is more important to know when not to adjust than when to adjust". In such cases Chiropractors will use some of their "non-force" techniques to restore movement and function to a faulty joint.
Do not feel disappointed if your Chiropractor tells you your bone is not "out of place". Excruciating pain can still be caused by locked joints, which are technically still in alignment. If you do not hear your joints go "crack" when they are treated do not feel cheated. The quickest way to full function and painless relief may sometimes be a silent one.