[scopendo]

I don't have the study, but I do have my own experience. About 5 years ago I was lucky to survive being hit by the propellor of a speedboat. I was struck across the sacrum, crushing nerves and fracturing both the sacrum and tailbones, as well as a deal of laceration for good measure. There is always a pressure, stiffness and dull spinal ache and occasional acute pain - though fortunately not too often for the latter.

Anyway, I find now that if I do any duration of impact-based activity that I will pay for it; I feel each impact in my spine, though that is bearable for a while, but the next day my lower spine and sacral area will be stiff and sore. Whereas when I swim there is neither immediate nor consequential pain relating to the injury.

It's lucky that I'm a lazy bastard and didn't intend on running anyway :p