EE WAS LIFTING A 33 LB CHOCK BLOCK WHICH WAS UNDER THE FRONT WHEEL OF HIS 40 TON CAT TRUCK. HE TURNED TO PUT THE BLOCK ON THE HOLDER LOCATED ON THE FRONT OF THE TRUCK AND HE FELT A SHARP PAIN IN HIS LOWER BACK. HE WORKED THE REST OF THE DAY. ON 4/13 HE WORKED UNTIL APPROX 10 AM AND THEN WAS SEEN AT A LOCAL CLINIC.