THE EMPLOYEE WAS PUTTING TROLLEY POLE OF A PORTAL BUS ONTO THE TROLLEY WIRE. FELT PAIN IN HIS BACK. LATER HE WAS LOADING BAGS OF ROCK DUST AND PAIN GOT WORSE. LIFTING 50 LB BAGS OF ROCK DUST. THERE WAS NO IMPAIRMENT OF MINING EQUIPMENT OR MINING OPERATIONS. INJURY DID NOT INVOLVE ANY COMPLIANCE WITH RULES.