Here is a simple game that lets you crouch and prone, the script is obviously not mine. However, i modified it ;).
You can use with blender 2.78 +, make sure to enable GLSL!!
http://www.mediafire.com/file/26o6jx42sf4ozy7/Crouch%20and%20Prone.rar
Voir sur youtube
Assets Pack Installation 
in Blender 4.3 Tutorial