zombies do not rise from the grave. The virus that causes human to become a zombie mutates the brain. If the human is already dead, the virus (borne through the blood) cannot spread to the brain, and even if injected directly into the brain, the brain cells are dead and would not respond to the mutations of the virus. The human must be alive of very freshly dead for the virus to be able to turn them. And seeing as zombies only have as much strength as the human who’s body it has did in life, its physically impossible for them to dig their way out of a coffin and 6 or more feet of earth.

-From The Zombie Survival Guide: By Max Brooks