INJURED WAS HELPING STACK 100 LB BAGS ONTO EXISTING PALLET FROM 54"" HIGH TO 72"". HYE STATED THERE WAS NO PAIN OR DISCOMFORT AT THAT TIME. HE ALSO WARNED THE NEXT SHIFT FRI NITE. STATED HIS BAC K MUSCLES FELT TIRED, BUT NOT HURTING. HIS FIRSTPAIN WAS MIDDAY.