Dystonia is an involuntary, sustained muscle contraction with abnormal posturing. Focal dystonia affects a part of the body. Writer’s cramp is a type of focal dystonia of the fingers, hand and/or forearm, it is specific to the task of writing, thus the name, and is the most common focal primary dystonic disorder.
It typically begins with an abnormally tight grip when a person writes and gradually progresses, making simple tasks such as holding a pen and writing very difficult.
Though initially thought to be associated with only writing later on it has been found that it is also associated with any particular task that a person performs repeatedly.
This could be a musician playing a musical instrument, Asst Surgeon performing surgery and his dystonia would occur when he holds the scalp, or a barber or a hairdresser while dressing of the head, etc.