WORKING ON AND OFF OF PLANK SPANNING THE BACKHOUSING OPENING. SHE WAS BREAKING UP HARD MATERIAL USING A PRY BAR TO BREAK UP A MASS OF CHUNKS. SHE WAS INVOLVED IN KEEPING THE CHUTE AND SCREW CL EAR OF CHUNKS. SHE COULD NOT IDENTIFY ONE SPECIFIC ACTION THAT CAUSED THE STRAIN. SHE DID NOT ECXPERIENCE PAIN UNTIL THE NEXT DAY.