I have a exam which involves lots of writing 50 pages 3 hr , nine times in 6 days. I can increase my speed by simply writing practice, but is there any Gym exercise so that my writing muscles will be stronger and enduring?
To be precise - you want better stamina? I would use powerball, or some other exercises that requires squeezing. Instead of short term - high intensity, use lower intensity, and train longer. Please observe your body, elbows a specially. You can develop golf elbow, and that is quite tricky to get rid of. You need to stop, and rest... and you have exam.
And agree that it can be done anywhere, not only at the gym. For instance you can buy something to squeeze, like de-stress ball, and squeeze it while watching TV...
Other from that. Famous violin players ware able to squeeze porcelain cups. So strong their fingers ware. Also talk with guitar players - they have strong fingers too. Problem is that writing requires more precision, and your pain comes from having muscles stretched to control pen.