Newton's First Law:
"When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force."
While Sir Isacc Newton was describing inanimate objects, this law is also true of humans and more topically, joints. With age and stress, the joints of our body break down. This process is often referred to as the Osteoarthritis, or more generically Arthritis. As joints become painful and inflamed, we move them less and they get stiffer. Along with the joints getting stiffer, the muscles weaken from disuse. The tendons become unhealthy and fragile and the ligaments tighten our joints even further. All of these processes conspire to "remain at rest". One of the mainstays of initial treatment for arthritis is physical therapy. The goal is to stem the tide of these degenerative processes with the hope of keeping us moving at a "constant velocity". While physical therapy is wonderful, it is not for everybody. There can be an expensive co-pay, the therapist's hours may not fit your life and the frequently traditional approach to disease and the human body may not fit your personality. Fortunately, there are alternatives! In a recent national meeting of the American College of Rheumatology, a series of studies were presented demonstrating the benefits of Tai Chi.
In a nicely done study, they found that a twice a week session with Tai Chi instruction was at least as good, if not better for patients with Osteoarthritis of the knees. It should be no surprise that a low impact exercise regimen that focuses on balance and strength, and includes life affirming meditation would be useful for people with knee arthritis. It has been demonstrated in the past to be useful in preventing hip fractures when use regularly in the elderly population. From these results, one could easily extrapolate that the practice of Tai Chi, or something similar would be good for people with arthritis of really any joint within the body.
There are many ways to say it: "Motion is Lotion", "People are like sharks, if they don't move, they die.", "Move it or Lose it!". The basic notion is the same, do what Newton recommends and stay in motion.