Reply by Ghidrah

What Dark_Lightning said, repetitive motion. I had a similar painful issue about 9 yrs ago, when I was doing a massive amount of entry and storm door installs. I believe my left thumb began locking on me because of the way I pushed the vinyl locking strips into the Andersen 2000, 3000 and 4000 series SDs, 3 or 4 weeks after I was done the problem stopped. I also believe I have the beginnings of osteoarthritis for at least the last 10 or more yrs.

-- I meant to do that!