make the player crawl to a height of 1 block

as you know, if you push a block onto the player's head with a piston, so that the player remains standing in a space of 1 block, the player's camera will lower, and the player will "lie down" and crawl. 
I need to cause the same effect.

more details: I'm making a monster crawling on the ground. when it catches up the player, the player must fall and be crushed by this monster and look at it from a bottom angle to see the mouth on the monster's stomach.
How can i do it?