EE WAS ASSIGNED TO REBUILD 9B CYC PUMP. IN DOING SO, THEY CHAINED OFF THE DRIVEN SHEAVE TO HOLD IT WHILE PULLING THE IMPELLER LOOSE WITH THE BRODERSON. AFTER THE CHAIN WAS ON, EE PULLED ON THE DRIVEN SHEAVE IN AN ATTEMPT TO TIGHTEN THE CHAIN. HE FELT A STRAIN TO HIS BACK. HE WAS EVALUATED BY A DR. LOST TIME STARTED ON 2/22/00.