A repetitive strain injury (RSI), also called cumulative trauma disorder (CTD), occupational overuse syndrome, or work related upper limb disorder (WRULD), is any of a loose group of conditions resulting from overuse of a tool, eg. computer, guitar or knife, or other activity that requires repeated movements. It is a syndrome that affects muscles, tendons and nerves in the hands, arms and upper back. The medically accepted condition in which it occurs is when muscles in these areas are kept tense for very long periods of time, due to poor posture and/or repetitive motions.
It is most common among assembly line and computer workers. Good posture, ergonomics and limiting time in stressful working conditions can help prevent or halt the progress of the disorder. It is also a problem for guitarists who play with very tensed muscles. Stretches, strengthening exercises, and biofeedback training to reduce neck and shoulder muscle tension can help heal existing disorders.Source: wikipedia