Hi, Kurt. I may be out of my league saying this, but FWIW, Bruce Lee had extreme near-sightedness, has one leg an inch shorter than the other, and yes, had a bad back. Of course, he was also in superb physical condition. Still, I've always been somewhat inspired by him in that if he can do it, I don't see why we can't. Then again, it is easy for me to say that since I'm not the one suffering from a bad back.
Personally, the way I look at it is that to become resolved in defending yourself is to make a choice in life that otherwise would have been made for you. If a BG is going to victimize us, they're going to victimize us regardless of what physical condition we are in. Bad back or not, if we have no choice but to fight, we will have to fight.
About knives. On a base level, my personal opinion about knives is that they are useful tools to have around. Almost everyday, I use them in some sort of menial tasks. Tactically, at the very least, the tool will provide you with a weapon advantage that might just be enough to save your bacon. Of course, it would be better to to rely on more than just that. So for more information, I recommend checking the sister site, http://www.bladeforums.com
I sympathize with your plight. Best wishes.
[This message has been edited by SB (edited 12-09-98).]