In osteoarthritis (rheumatoid arthritis is a different story) there is inflammation in a bony joint with pain and swelling, sometimes resulting in changes to the bone.
Joints have nerve endings that tell the nervous system what the joint is doing. Those nerves activate when the pressure in the joint is excessive. Prolonged pressure compromises the tissues within the joint that are there for protection. Various structures cushion the bones from rubbing on each other (like the discs in the spine) or provide lubrication and nutrients (like bursas and synovial membranes in joints like the knee) or provide strength and support (like ligaments and tendons in the shoulder). Over time the pressure causes wearing of joint structures which breaks them down or makes them thin and dry. Both situations result in increased friction that causes inflammation which forms acids that must be buffered to prevent cell damage. When there aren’t enough buffers to neutralize the acids, the body can take calcium from its bones to do the job (Wikipedia). The bone degenerates from both the wearing and the loss of calcium and becomes distorted by the action of osteophytes.
So, to treat arthritis, it makes sense to take the pressure off the joint so it can move freely. The mechanical pressure and inflammation will then be reduced and the joint can receive nutrients, remove the waste products that cause pain, and regain its health.
Reducing inflammation is important, but it will return if the pressure is not relieved. This involves stretching and treating the muscles that cross the joint and move it. When muscles are tight the joint will be tight, when they are are flexible, the joint can be healthy. In my clinic outside of Boston, I use Active Isolated Stretching and Neuromuscular Therapy as the most effective treatment along with applications of ice to the joint itself.
Exercise that tightens the muscles can cause more problems in the beginning, so manual treatment and stretching are necessary to take advantage of the well-known benefits of exercise. When health has been restored to the joint, a careful program of incremental increases in weight and intensity of strength training can be undertaken.