The British Association for Performing Arts Medicine in York, England, hosted the 1997 Health and the Musician conference. A paper by Christine Zaza, PhD, presented a review of research of musculoskeletal disorders in musicians. One conclusion was "The prevalence of PRMDs [playing-related musculoskeletal disorders] in musicians is consistent with the prevalence of work-related musculoskeletal disorders for other workers."
In short, playing an instrument is generally no more or less harmful than other common repetitive tasks. There is, however, considerable evidence that guitar and piano playing, which involves low-impact movement and moderate effort, can maintain and even improve flexibility when a disorder is present.