I've been competitive kayaking since the age of 14 (now 46) and my back is pretty much screwed, largely due to having very little lower back arch from sitting down paddling my whole life. I was in pretty much constant pain with some highs and lows

Took up SUP paddling last year. If I SUP paddle 2-3 times a week I no longer have a back issue at all. I also still paddle once a week.

So I guess in answer to your question, yes sit down paddling can contribute to back issues over a long period of time however I believe from my experience at least that this would not be the case with SUP paddling, although who knows if that will be the same for shoulders and wrists etc.

Thank's for your input guys. I always think proper technique and a balanced stretching regime goes a long way to overcome most of these issues.

Hope young Mr Baxter's got some good recovery stretching program given that unusual technique of his and his age