Muscle‐intensive and High‐Impact Exercises Differentially Influence Whole Bone 3D Morphology in Young Outbred Male Mice

Abstract

Skeletal morphology is predominantly dictated by the genetic background and the habitual loading environment of individual bones. Mechanical loads placed on bones are derived from intracorporeal contractions of muscles and from extracorporeal ground reaction forces generated by bearing weight and by substrate reaction forces during locomotion. We investigated the relative effects of these two sources of load on the growing skeleton using an outbred mouse model system. The 3D shape of the femur was compared across groups of mice following discrete exercise regimens designed to emphasize specific loading environments. Hsd:ICR (CD‐1) mice (N=80; 20 per group) were randomly assigned to one of four treatment groups at four weeks old: swimming which emphasizes muscle forces, dropping which emphasizes impact forces, wheel running which results in a combination of muscle and impact forces, and non‐exercised controls which were free to move about their cages. Durations of swimming, running, and impact loading were matched so that mice received approximately the same intensity. After 20 days mice were euthanized and right hind limbs were micro‐CT scanned (~20 μm3 voxel size). Femoral shape was defined by digitizing 23 landmarks on each scan. To adjust for size, each individual femur was scaled to the geometric mean of all possible linear distances among landmarks. Group differences were analyzed using Euclidean Distance Matrix Analysis (EDMA), and differences >5% were considered substantial. In all three activity groups, we find decreased robusticity of the femoral diaphysis and increased distal condylar breadth compared to non‐exercised controls. Additionally, each group displays unique femoral morphologies when compared to controls. The swimming treatment group shows increased length of the femoral neck. The wheel running treatment group shows a narrower femoral neck with a larger femoral head. The impact loading group shows a lengthened and reoriented distal diaphysis. Overall, the running and swimming groups show similar patterns of morphology that diverge from the non‐exercised mice, suggesting muscle forces differentially affect the loading environment during running in mice compared to impact forces. Further, these results indicate the regional sensitivity of femoral morphology to muscle‐ and impact‐derived loads, and emphasize how activity patterns during adolescence can lead to distinct morphologies in the limb skeleton.Support or Funding InformationResearch support: University of Missouri School of MedicineThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.