EE WAS IN A MANLIFT INSTALLING 6"" X 15' PIPE THAT WAS SUSPENDED BY A NYLON SLING ON A PITMAN BOOM TRUCK. TRUCK COULD NOT BOOM OUT BECAUSE OF OVERHEAD INTERFERANCE. EE & FOREMAN WERE PUSHING ON PIPE TO DRIFT INTO PLACE AS THEY BOOMED DOWN LOAD. PIPE MOVED TOWARD EE WHICH PRESSED HIS BACK INTO HANDRAIL OF MANLIFT CAUSING MUSCLE STRAIN MID-BACK.