Well as a knife instructor, I can tell you that this is a tough question, but I'll try my best.
If you are in a wheel chair the best you can do is get away, prevention and awareness are your best weapons. If you are trained in knife fighting then you know what to carry; if trained in firearms, well, you know. I really do not reccomend to try to disarm a knife especially from a trained fighter. I tried this last week with my advanced students class and myself was only successfull only 50% of the times, the rest of the times I either dissarmed the knife but got cut or didn't dissarmed the knife at all and got cut.
If you are unarmed and have to dissarm the attacker (assuming you have no other choice) the last thing you want is to go to the ground. A flailing opponent on the ground will cut you many times before you can control him; I don't care how good of a grappler you are, you will get cut, possibly fatally. Another thing, if an assailant is going to use a knife against you, more than likely he'll stab at your face or midsection, your legs are a bit of a low target, he'll go for the easiest one; what's at reach. Also, do not worry too much about this, except at certain ponts, the legs are largely muscle and can take damage better. Worry about the tendons, ligaments, veins and arteries located at key points in the human anatomy http://www.innerbody.com/image/cardov.html
. Those are dangerous if cut.
As for a dissarm, just keep your guard up and try to protect your arms with a heavy piece of clothing, if you get a grip on the knife (even the blade itself) or knife arm, DON'T LET GO! Apply any technique you want but make it count, no second chances here. Go for broke and fight dirty, no shame on that if you life is on the line.
It is all a matter of how bad you want to live and fight back.
Yours in the Spirit of the Warrior-Scholar,
Carlos M. Roman H.I., American Kenpo Karate Studios