Even if you have spasms you still 'want' to. Your broken muscles/nerves want to.

Your subconscious wants to.

Also, fun fact.
The feeling that you hit the ground or perhaps witness something in the dream that causes you to have a huge body spasm is actually your heart rate dropping so fast that your brain thinks you're dying so it's like "Bitch wake yo ass up." and forces your muscles to all extend/ contract at once in an attempt to wake you up.