It is no secret to just about any long term pianist that occasional pain and muscular discomfort may occur from time to time, during their playing careers. Effectively, this is the result of repetitive strain injury. Many serious or professional pianists will typically practice for around seven hours a day, and over a period of a number of years, will add up to a long term pressure on the joints and muscles of the arm, hand and wrist.
As with any sporting activity, it is encouraged to ensure that a thorough warm up session be undertaken first, and then too, a cool down session, to slowly relax the body prior to the cessation of exercise. The same can be applied for pianists, as this will contribute to less chance of injury. Now, I am not going to suggest that playing scales and arpeggios before you launch into the Beethoven Pathétique Sonata is going to cover you against all and sundry injuries. However, it certainly is an important part of your playing regimen.
However, let's consider the actual forces and strain that are going on with the arm, and hands, let alone the wrist and fingers. As far as the arm is concerned, many pianists extend the arm in such a way as to apply considerable force to the wrists ad tendons, as they play, and this can certainly be sustained for a period of time,. However, given the nature of the work in question, some pianists will start to notice pain if they are stretching their fingers considerably (when the hands are small, and the notes spread widely over the keyboard). The preceding example is by no means the only possible scenario, but certainly a common one, amongst others.
There are a number of possibilities to undertake to resolve the problem, and for some, that includes attending physiotherapy clinics. They certainly have their place, and some are undoubtedly very good. In fact, some have an excellent reputation. A question that I would ask however, is to perhaps see if we can find a way to eliminate the problem form developing in the first place. Is such an option available to pianists? The short answer is yes, although it is dependent on the teacher, and student adopting a different method of playing, which like any new endeavour, will take time to master, but ultimately, bring forth greater rewards to the pianist, as well as the pleasure of an improved sound. You might well ask, "How can dealing with tendonitis improve the sound?"
The sound itself, is the result of the quality of the playing. When a pianist is able to play with less fear due to a greater relaxation at the keyboard, and a technique that allows a greater freedom of movement in the hand and fingers, as well as positioning relative to the keyboard, the strain can be greatly minimised on the tendons. Whilst some people may try to convince you that you can cure the problem in a few days, I personally believe that to be irresponsible, and dangerous. If you play on through a damaged tendon, you will likely do more damage. You are better off resting the injury till it heals, and then applying a technique that has been proven, like the Bowman method.