EE WAS PACKAGING 50# BAGS OF PRODUCT WHEN HE FELT A SHARP PAIN IN UPPER MIDDLE SECTION OF HIS BACK. HE SAID HIS BACK POPPED AT THE TIME & THAT HE FELT THE PAIN FROM THE MIDDLE OF HIS BACK UP T O THE LEFT SHOULDER BLADE. THE PHYSICIAN'S DIAGNOSIS WAS A ""THORACIC STRAIN."" HE TOLD EE TO TAKE A DAY OFF & THEN RE-EXAMINED HIM NEXT WORKING DAY (11/4) & RELEASED HIM FROM RESTRICTED DUTY.