Effects of activity and selection on mouse femoral strength

Bone is a hard tissue providing mechanical support to vertebrates and is influenced by both genetic and environmental factors. Stress from mechanical loading causes micro‐fractures in bones, triggering dynamic bone remodeling by shifting the balance of absorption and formation to net formation. Prolonged inactivity can lead to bone dystrophy, which reduces bone strength. In order to assess how genetic and environmental factors can independently affect the structure of bone, we used mice from lines that have been selectively bred for high levels of voluntary wheel running (High Runner, or HR lines) that had access to cage wheels throughout their lives. Femora from HR and control mice were dissected and defleshed in order to perform material strength testing on the bone. A custom‐made rig was designed to hold the femora in place as a materials testing system (MTS) was used to perform a three‐point bending test at the mid‐diaphysis of each individual femur. We recorded morphometric data prior to testing and the modulus (stiffness) and failure stress were calculated from the MTS tests. HR mice have thicker femora, but are lighter in density, than control mice. HR mice with wheel access had longer than average femora, but stiffness and failure strength was similar among all groups.