Marlena clearly seems to ‘burst’ from the torso if you watch the clip in slow motion. Also, when the army wheeled a soldier past on a stretcher minutes earlier (with the torn open, empty carcass) you clearly hear them say “we’ve got a bite”, indicating that the result of being bitten is some form of explosive force from within the chest.
http://youtube.com/watch?v=2jte-PRrmgo&feature=related
I heard from someone that it would be medically possible for, if your heart somehow managed to well up with pressure, to cause an explosive force that could tear open your lower body. Don’t quote me on that though, I didn’t find anything to confirm it.
@uberbatman: Cloverfield is less about why anything happens, and more about what would happen if something happened. Stop asking questions; answers are a bonus, but is it not possible to just enjoy the film on a simple “if that happened to me” level?