Repetitive activities of the upper extremities—such as sports activities, playing musical instruments, or work activities that require repetition over the course of six or more hours per day—can result in a variety of problems in the upper extremities.  These problems are termed cumulative trauma, or repetitive-stress injuries.
The exact cause of these problems is unclear, but they are thought to result from repeated very minor trauma to muscles, tendons, and joints within the extremity, resulting in some swelling or inflammation.
The manifestations of cumulative trauma can be quite varied, resulting in conditions ranging from tendonitis problems (such as tennis elbow or golfer's elbow) to de Quervain's disease, to problems associated with synovitis, to an array of nerve-entrapment syndromes: