What happens to your muscles during rem sleep?

They are paralyzed: During rem sleep, all the voluntary muscles, with the exception of the diagphragm, eye muscles and middle ear muscles, are inhibited to the point of paralysis. A teleological explanation could be that this happens so that we will not act out our dreams. Rem behavior disorder occurs when this inhibition fails and dreams then are acted out, often causing injury to the dreamer or bedmate.