THREE EMPLOYEES WERE MOVING PARKING LOGS AROUND THE YARD. THEY HAD NEW CHAIN (15 FT) TIED TO THE BUCKET. THE CHAIN LOST ITS SLACK ON ONE SIDE OFTHE BUCKET, SO EMPLOYEE BENT DOWN TO GET THE RIG HT AMOUNT OF SLACK IN THE CHAIN. WHEN HE STOOD UP IS WHEN HE FELT A POP IN HIS BACK.