What is Manipulation?
Spinal manipulation, or just manipulation, is practiced by specialists such as chiropractors and osteopathic physicians to improve the range of motion of joints that have become stiff, due to injury or time. Manipulation is defined as "an assisted passive motion applied to the spinal facet joints and sacroiliac joints," meaning that the patient is passive and the manipulator assists. Though most patients go to chiropractors to get treated for stiff joints and muscles of the back, manipulation treatments can be applied to any stiff joints of the body.
Simple put, patients requiring adjustments from chiropractors have stiff joints that are manipulated to make it move. Manipulation techniques can unlock stiff joints to reduce their range of motion. A less mobile joint is less likely to be painful than a stiff joint. This principle is true for most joints, including those in the elbow wrist, and ankle.
Our own body has several feedback mechanisms that allow us a certain range of motion; and our body provides us with feedback mechanisms in the form of pain when we are bending or flexing beyond what would be healthy for our bones and joints. While this feedback mechanism is usually appropriate to protect us from moving twisting and bending to the point of injury, the mechanism can also work against us to make us feel pain unnecessarily. Toward the and of a joint's normal range of physiological range of motion, there is a buffer zone. At the end of that buffer zone, there is an elastic barrier of resistance. This barrier, has what back specialists call a "springlike end feel" which is the result of negative pressure within the joint capsule. (This negative pressure is one of the mechanisms that help stabilize the joint. Muscles, ligaments and the capsule itself are the others.)
If the joint surfaces are forced beyond this elastic barrier by a manipulator, they move apart with a cracking sound, entering what is called the paraphysiological ROM. This constitutes manipulation. Mobilization, on the other hand, stays on the buffer zone side of the elastic barrier within the joint's normal physiological range of motion.
The cracking sound and pressure you feel then a back specialist performs a manipulation results from the sudden liberation of gases that are contained in the synovial fluid of the joint's capsule. Doctors and clinicians specializing in joint problems call the phenomenon cavitation. These gases are usually reabsorbed by the same joints giving off these gases within a few minutes of the manipulation. For approximately 30 minutes, however, the elastic barrier of resistance between the buffer zone and the paraphysiological ROM is absent. During this time, there is an increase in the joint space and the joint is somewhat unstable, making further manipulations unsafe. After the gases are reabsorbed, the negative pressure returns to normal. The joint space narrows and the elastic barrier of resistance between the buffer zone and the paraphysiological zones are re-established.
The manipulations may serve other purposes other than just increasing the range of motion of the joints. Some doctors think that by manipulating a joint, the large afferent nerve fibers can be stimulated, inhibiting the small afferent "pain" fibers from transmitting pain.