The roof bolt crew finished bolting the face of #6. The miners backed out of the face and turned the machine off when a rib rolled off from behind the line curtain. The rib measured 6' L, 8"" W, and 20 "" H. The rib struck the miner in the lower back/legs. The miner had pre-existing back problems and was diagnosed with a lumbar strain. Medical was not released until 8-18-2009.