Quad muscles not firing, unable to do leg raise or lift foot from bent sitting position

[Judith Mollet]

there seems to be alot of information missing about what sort of accident you had and how your state of "back health" and pain back and leg mobility was beforehand. If your quads aren't firing the knee will give way unless braced or hyperextended. Your doctors will have to sort out if your lack of quads is due to nerve injury somewhere along the line or extensive tearing (you don't say how much) and scarring of muscle tissue or pain inhibition due to knee injury or a combination of a few causes.