EMPLOYEE WAS PERFORMING MAINTENANCE ON CRUSHER SCREEN WITH THE ASSISTANCE OF A CRANE & OPERATOR. EE WAS POSITIONING A PLATE ON THE SCREEN WHEN THE OPERATOR BEGAN LOWERING THE CRANE BOOM ON SIGNAL. THE BOOM MOVED FASTER THAN THE EE ANTICIPATED & HE STRAINED HIS BACK IN ALTERING HIS POSITION. EE FINISHED HIS SHIFT & SOUGHT DR.'S CARE ON SATURDAY, 1/14/06.