UNIT SUPERVISOR WAS MARKING FACE. SCOOP OPR. WAS ASSIGNED TO CLEAN #5 ENTRY. OPR WENT THRU CURTAIN 80'OUTBY ACCIDENT AREA & WAS PREPARING TO DUMP BUCKET LOAD AT FACE OF #5 WHEN HE HEARD SOMEON E YELL. OPR IMMEDIATELY BACKED UP WHEN HE DISCOVERED HE HAD STRUCK SOMEONE W/BUCKET. SUPERVISOR SUSTAINED INJURIES TO BOTH LEGS BELOW THE KNEES.