An employee was conducting maintenance on an obstacle course. He was securing the bottom section of a two-part extension ladder to a tree by screwing bolts into the tree. He began ascending the ladder again with a blue strap to secure the ladder to the tree when the ladder began sliding on the tree and he fell approximately 8 feet to the ground. The employee sustained fractures to a rib on the left side, his sacrum, and three parts of his pelvis, as well as abrasions.