Or the blood is all coagulated so nothing is even going to pump to move the muscles which move to bones. Or better yet, because 10 minutes without oxygen creates necrosis in the brain and permanent damage, no signals will fire off to do anything.
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If it’s a slightly later zombie which is a reanimated corpse, it’s gonna vary from body to body. If the body’s been buried for a while, don’t worry, the tissue damage will make it so that the muscles can’t even contract to move the brittle bones.