Effects or robots
i say that because no human can bend their legs the way aliens does
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plus they are very tall.
also triing t pefect the whole running thing(Runner) wouldnt be hard since they stad like dogs and mainly in reality the human arms are shorter than the human legs.
the mandables which hides inside the alien's mouth cant really be done without some type of xcontroller and controlling the tail is almsot impossible without very loose and very flexable wires and components
lets put an example....like the predator,it stands so tall not even the average male adult can fit it and the strengh and meight of the armour cant be weilded at any circumstance.lets not forget the Visions they have.