RSI (Repetitive Strain Injury)
RSI is a very common problem and a potential risk for all types of musicians, with the possible exception of singers. It is a work-related overuse injury that usually (although not always) affects upper areas of the body, and musicians are at increased risk of this as practising, especially at a high level, involves the protracted repetition of fine movements. Symptoms include pain and tenderness, throbbing, weakness, tingling, cramp and stiffness, and although they may be mild at first, they can become extremely severe if not acted on promptly and if the musician tries to play on through the pain. Players are often reluctant to admit to having an injury due to the competitive nature of the music world and can often present late, and RSI can be career-threatening if not taken seriously and treated promptly. Good technique and taking care to warm up and warm down properly when playing can help to prevent the problem, and keeping as relaxed as possible when playing also helps. Overuse of computers and mobile phones can also predispose musicians to RSI.
Commonly affected areas are the wrists, forearms, elbows, back and neck. 
Certain instrumentalists are at particular risk of this condition. Violinists and violists can develop RSI in their bowing arm, and keeping the arm relaxed and the wrist fluid helps, while pianists can develop problems due to poor posture. Some very technical pieces with particularly difficult repetitive movements can bring on problems, and very advanced young players can run into trouble when taking on virtuosic pieces for which their bodies are not yet ready or when over-practising. Drummers tend to have problems with the back and the wrists, and the Moeller method (a percussive stroke method used in drumming which is thought to require significantly less effort and to carry less risk of injury than other methods) can be helpful in developing a more relaxed style of playing. Guitarists can develop RSI of the fingers, and low-slung guitars can also cause problems.
On developing symptoms it is crucial to stop playing and for the performer to seek health advice. Rest is paramount and treatment options include NSAIDs, hot and cold packs, supports, physiotherapy, osteopathy, and acupuncture. Change in playing technique and instrument set up may also be needed. It is very helpful for musicians to bring their instruments to appointments with their therapist so that their playing position can be demonstrated, and this can help with solving the problem.
Measures that musicians can take to try to prevent developing RSI include good playing technique, avoiding over-ambitious repertoire and over-practising, improving posture, staying relaxed when playing, doing warm up and warm down exercises before practising and performing, and taking regular breaks. Exercise is important and yoga and Tai Chi may be especially beneficial, while many musicians use the Alexander Technique (an educational process which was created to retrain habitual patterns of movement and posture) to very good effect.