The legs were ereased by computer.
On Sat, 23 Jul 1994, Abigail Miriam Feder wrote:
> One effect from Forrest Gump I haven't heard discussed anywhere -- Gary
>  missing legs.  I know on stage such an effect is often achieved by actually
>  taping the calf to the thigh at the knee, but in most of the shots of Sinise,
>  the thighs looked too thin to have a calf taped to them.  In the scene in
>  he kicks the prostitutes out of his room, I thought I detected a flicker as
>  hoisted himself back into the chair.  Was the double amputation a special
>  effect and if so does anyone know how it was done?
>         --Abby