Exercise-induced changes in the cortical bone of growing mice are bone- and gender-specific

Abstract

Fracture risk and mechanical competence of bone are functions of bone mass and tissue quality, which in turn are dependent on the bone's mechanical environment. Male mice have a greater response to non-weight-bearing exercise than females, resulting in larger, stronger bones compared with control animals. The aim of this study was to test the hypothesis that short-term weight-bearing running during growth (21 days starting at 8 weeks of age; 30 min/day; 12 m/min; 5 degrees incline; 7 days/week) would similarly have a greater impact on cross-sectional geometry and mechanical competence in the femora and tibiae of male mice versus females. Based on the orientation of the legs during running and the proximity of the tibia to the point of impact, this response was hypothesized to be greatest in the tibia. Exercise-related changes relative to controls were assayed by four-point bending tests, while volumetric bone mineral density and cross-sectional geometry were also assessed. The response to running was bone- and gender-specific, with male tibiae demonstrating the greatest effects. In male tibiae, periosteal perimeter, endocortical perimeter, cortical area, medial-lateral width and bending moment of inertia increased versus control mice suggesting that while growth is occurring in these mice between 8 and 11 weeks of age, exercise accelerated this growth resulting in a greater increase in bone tissue over the 3 weeks of the study. Exercise increased tissue-level strain-to-failure and structural post-yield deformation in the male tibiae, but these post-yield benefits came at the expense of decreased yield deformation, structural and tissue-level yield strength and tissue-level ultimate strength. These results suggest that exercise superimposed upon growth accelerated growth-related increases in tibial cross-sectional dimensions. Exercise also influenced the quality of this forming bone, significantly impacting structural and tissue-level mechanical properties.

The bone diagnostic instrument II: Indentation distance increase

“…Mechanical properties were determined by monotonic loading to failure under four-point bending. 35 Four-point bending develops a constant bending moment between the inner loading points. The advantage of this technique is that a local weakness in the testing region will be discovered by causing failure at that site.…”

“…Force and displacement data were recorded during the test and then converted to stress and strain using beam bending theory and caliper measurements from the fracture site. A custom designed MATLAB program ͑The MathWorks, Inc.͒ was used to perform this transformation and calculate all mechanical properties 35 ͒. The result of the four-point bending tests was that the aged bone had, at the 0.01 level of significance, smaller yield stress, ultimate stress, failure stress, and toughness relative to the younger bones.…”

Collagen and mineral deposition in rabbit cortical bone during maturation and growth: Effects on tissue properties

“…Evidence exists that gender affects cortical bone growth rate and adaptation to exercise. 37 However, we cannot draw any conclusions on gender effects. The rabbits in our study become skeletally mature between 6-9 months of age, in line with a recent report that demonstrated negligible bone growth in rabbits after 6-8 months, while articular cartilage showed mature structure by 3-4 months.…”

Exercise prevents β-aminopropionitrile-induced morphological changes to type I collagen in murine bone

“…Mice experienced either Sed or Ex previously shown to alter bone structural and tissue-level mechanical properties. 35 All mice had access to food, water and cage activity ad libitum. Ex animals ran on a treadmill (Animal Treadmill: Exer 3/6; Columbus Instruments, Columbus, OH, USA) for 30 min per day at 12 m min À 1 on a 51 incline for 21 consecutive days.…”