Ditto. While my back doesn't bother me, a few x-rays several years ago showed what my doctor said was "arthritis like an old man". Asked if I'd had any previous back injuries, I said no and then asked if repeated landings in a military 'chute would do it. Doc looked at me as if I were stupid and said "uh, yeah" and advised me to never do it again for fear of further damage and aggravating what arthritis is already there.

Sport parachutes are considerably slower in descent than military chutes. Landings are softer, too. Or so I'm told. I never jumped a sport chute.