Exercise-induced loading increases ilium cortical area in a selectively bred mouse model

Objectives: Little is known about how ilium cortical bone responds to loading. Using a mouse model, this study presents data testing the hypothesis that iliac cross-sectional properties are altered in response to increased activity. Materials and Methods: The sample derives from lines of High Runner (HR) mice bred for increased wheel-running activity. Four treatment groups of female mice were tested: non-selected control lines housed without (N = 19) and with wheels (N = 20), and HR mice housed without (N = 17) and with wheels (N = 18) for 13 weeks beginning at weaning. Each pelvis was μCT-scanned, cross-sectional properties (cortical area—Ct.Ar, total area—Tt.Ar, polar moment of area, and polar section modulus) were determined from the ilium midshaft, and robusticity indices (ratio of the square root of Ct.Ar or Tt.Ar to caudal ilium length) were calculated. Mixed models were implemented with linetype, wheel access, and presence of the mini-muscle phenotype as fixed effects, replicate line nested within linetype as a random effect, and body mass as a covariate. Results: Results demonstrate that the mouse ilium morphologically resembles a long bone in cross section. Body mass and the mini-muscle phenotype were significant predictors of iliac cross-sectional properties. Wheel access only had a statistically significant effect on Ct.Ar and its robusticity index, with greater values in mice with wheel access. Discussion: These results suggest that voluntary exercise increases cortical area, but does not otherwise strengthen the ilium in these mice, corroborating previous studies on the effect of increased wheel-running activity on femoral and humeral cross-sectional properties in these mice.