Hard to say the cause and effect relationship, but after 10-12 years of
heavy paddling in trippers, with all those 200lb portages and hours and
hours pushing a thousand-pound boat with just shoulder rotation, I now
have a chronic bad back that can be debilitating. I have nothing but
anecdotal evidence, but I have no personal doubt that hundreds of
thousands of twists on my lower back (efficient paddle strokes means
you rotate your shoulders), combined with the pounding on my spine when
tumping or portaging, has been the primary cause. I suppose the
unbalanced diet of a river tripper (low on fresh veggies, high on meat
and canned goods) probably had its impact, too.

Age certainly plays a role, but I was perennially young far longer than
my friends until my back went out. Now I have seemed to have aged
beyond them, as I am less able to hike up big hills, sleep on the
ground for extended periods, or take off on a run or hike without
serious consideration of 'plan B', in case my back goes out.
--riverman