"It's always good to test an effect with the actor who will be wearing it. Sigourney was great about giving us access to her time, but when we wanted to tweak these Queen Chestburster rigs, we chose to test on the crew member with the most ladylike hair. Thank you, future Oscar winner Mark Coulier.
The spreading of the ribcage was accomplished with "air muscles". Lengths of high pressure water hose were folded in half then filled with compressed air, causing this "muscle" to unfold. The advantage was that the mechanism, unlike Mark's hair, could be very low relief, like Sigourney's shaved head."