An employee was tightening a bolt on the final assembly of a stump puller. The wheels were separated and resting on 6-inch steel blocks with the mainframe on steel I-beams. The employee stepped between the wheels to adjust the spacer plates and used his legs to separate the wheels. When the stump puller was raised slightly and the weight came off the support beams and blocks, it rolled backward a few inches, causing the lifting wheels to close and catch his left leg between the wheels. The employee sustained a left tibia fracture, a torn calf, and bruising to the lower extremities.