Did anyone else notice this? My thumb joint is swollen and first I thought it was bacause I had fallen on my paddle, but I noticed that sudden bracing can put a very stress on the thumb, more so than a low brace in a kayak.

I think I found out the cause and trick to avoid this.

I typically keep the grip of my bottom hand extremely loose, which saves some energy, but when you fall forwards and have to brace, your full body weight lands on an extended thumb.

So keep a tight grip on the shaft in whitewater and your thumb should stay fine

Yes WW SUP is very hard on the thumbs. I have had some serious issues with my lower hand thumb from bracing so hard I am partially dislocating it because I am hanging on for so long and basically ripping it off trying not to fall because falling in the river usually hurts. I have developed a technique where I dont wrap my thumb around the shaft at all above big rapids and hard maneuvers so that if I fall I dont get hung up and re-injure it. At this point I have develope inflamatory arthritis in the thumb but now its getting better with the no thumb wrap around the shaft technique. Basically my thumb rests right next to pointer finger whenever I am in fear of some very hard bracing.