From the first year in school, I really forced myself to do these alot..as well as many character studies.
I use about 5 sketchbooks in a term on top of school works and 3 hours of lifedrawing 7 times a week. And it's actually 9 times now..
Anyways..ya this is the reason of my hand injury, as well as not taking too much attention to my health. But it can be avoided doing proper stretch and exercise. I've been going to Athletic Therapy Centre these days, and it's alot of help.