To expand (pardon the pun), muscles work by contracting when electricity goes to them. Your nerves send electrical signals, and the muscles pull. If you constantly pass electricity through them, they'll stay pulled, and you won't be able to relax them to be able to breathe.

The heart is a muscle. Chambers of it expand and fill with blood, then contract (thanks to a wee electrical timer) and fire blood around the body. Rogue electric signals can mess with this timing and can make it stop.