EE WAS HELPING SHOVEL OPERATOR APPLY GREASE ON P&H 4100 SHOVEL STICK WHERE WELDING REPAIRS HAD BEEN MADE. WAS USING 5-GAL BUCKET OF GREASE WHEN HE BENT OVER & OVEREXTENDED RESULTING IN PAIN TO LOWER BACK. CONTINUED TO WORK REMAINDER OF SHIFT & WORKED FOLLOWING DAY. DIDN'T REPORT TO WORK JUNE 9 DUE TO BACK PAIN. SUFFERED MUSCLE SPASMS @ HOME ON 9TH & WENT TO HOSPITAL BY AMBULANCE.