Osteoarthritis is the most common form of arthritis. This is the form that usually comes with age and most often affects the fingers, knees, and hips. Sometimes osteoarthritis follows an injury to a joint. For example, a young person might hurt his knee badly playing soccer. Then, years after the knee has apparently healed, he might get arthritis in his knee joint.

Muscle is a contractile form of tissue. It is one of the four major tissue types, the other three being epithelium, connective tissue and nervous tissue. Muscle contraction is used to move parts of the body, as well as to move substances within the body.