WORKER DISMOUNTED LOADER TO THROW LOOSE ROCK IN CRUSHER. HE BENT AND TWISTED AT WAIST TO PICK IT UP, THEN TWISTED AT THE WAIST TO THROW THE ROCK, STRAINING/SPRAINING LOWER BACK. HE DID NOT FEEL DISCOMFORT UNTIL LATER THAT EVENING AND REPORTED IT ON TUESDAY, 6 JAN. HE WAS SENT TO CLINIC FOR TREATMENT. WORK LIMITATIONS WITHIN DUTIES AS OPERATOR. NO LOST TIME.