EE WAS HELPING MOUNT AN SAFETY LADDER CAGE ON A STRAIGHT LADDER AT THE EXPANDER TO BE STABILIZED, ASSISTED BY 2 EMPLOYEES STANDING AT GROUND LEVEL. HE FELT A SHARP TWINGE BETWEEN HIS SHOULDER BLADES. HE WAS HOLDING THE UNIT HALFWAY FROM HISBODY. AFTER HE FELT A TWING, HE HELD THE POSITION TILL UNIT WAS SECURED.