Inj EE & another EE were changing positioin of flop gate at feed end of 6x16 AC screen. Gate consists of a plate attached to a shaft. Gate weighs 100lbs. They tried to reposition gate, shaft broke. EE tried to hold weight of gate. Caused back pain EE went to chiropractor on 1/23/06 has possible compression frac. Excused fro work tl 1/30/06, follow up w/Dr 1/27/206