Among the books and binders in her office in the Mechanical Engineering building, associate professor Heidi-Lynn Ploeg?s shelves are filled with bones. She pulls out a thin cardboard sleeve, and inside are dozens of mouse femurs. Each one of these leg bones is shorter than the length of a fingernail. ?It?s amazing how we can compare these to the human body,? Ploeg said. Ploeg is the director of a Bone and Joint Biomechanics laboratory at the University of Wisconsin-Madison.