The muscles in your body need ATP (adenosine triphosphate) in order to relax. But ATP requires oxygen. No more breathing means no more oxygen, which means no more ATP, which means the muscles seize up and can’t relax. This chemical change, collectively called rigor mortis, starts around your eyelids and jaw and spreads through every muscle in the body, even the organs. Rigor mortis makes the muscles incredibly stiff.
