So, MY thumbs, particularly the right hand one, which once upon a time got broken and torn ligaments, so isn't quite as mobile as the left one, get very sore when:

I ride down "MY" hill, which needs me to ride the brakes too much -- the reach is a bit much, and I tend to put more weight/pressure on my hands when descending.

Hurts like a bugger when I've been doing this, but it gets better if I avoid that particular ride for a while, until the next time I do it.

When it bothers me, cuz I'm using that hill for practice, so I need to come down it after I go up it, of course, I wear my bike glove on that hand while sleeping -- it keeps my hand warm, and supports the sore thumb. I could probably get some sort of splint for those painful days, but the glove is working for me, and I've already got it -- don't need to spend money on something else...

Soon, I'll find a few hours to go spend time with my LBS guys, and have something done with my brakes to make them fit my hands better! It'll also help to get better at riding down my hill -- so I'll practice, y'know?

Karen in Boise