Employee was operating the #18 roof bolter at the face of crosscut #3 to #2 entry in unit #2. He was installing a 4' roof bolt when it bent as he was pushing it up in the roof using his drill pot. He then tried to pull or tug on the bolt by hand to pull it out of the roof so a new bolt could be installed to replace it. At this time he felt a pull in his lower back near his belt.