I thought of this while in my English class today. I was writing as a part of a journal assignment my professor gave us, and my hand started cramping; the question came to mind of how a monk in the middle ages would deal with this pain if they have to transcribe entire books. If there any record of how they addressed this issue? I imagine the pain must have been unbearable.
Sheer brute willpower. Monks felt the same pains you or I might feel, aggravated by factors such as a lack of artificial light or heat that would drive most of us to distraction. They persevered because it was simply what they were required or expected to do. This does not mean they did it without complaint, however.
Lapham's Quarterly published an article on marginalia from monks found on illuminated manuscripts, containing complaints about such things as the pain, the conditions, and the fear that their hard work would be unrecognized. It looks like some of the content is behind a paywall, but thankfully some samples have been excerpted by others and can be read for free.
Sorry if this is a better question for /r/shittyaskhistorians