Exercise alters mineral and matrix composition in the absence of adding new bone

The mechanical properties of bone are dictated by its amount, distribution and 'quality'. The composition of the mineral and matrix phases is integral to defining 'bone quality'. Exercise can potentially increase resistance to fracture, yet the effects of exercise on skeletal fragility, and how alterations in fragility are modulated by the amount, distribution and composition of bone, are unknown. In this investigation, the effects of exercise on the size, composition, mechanical properties and damage resistance of bones from mice of various ages, background strains and genetic makeup were assessed, as a means of testing the hypothesis that mechanical loading can improve skeletal fragility via compositional alterations. C57BL/6 mice (4-month-old males) ran on a treadmill for 21 days. Tibiae from exercised and control mice were analyzed for cross-sectional geometry, mechanical properties, microdamage and composition. Exercise significantly increased strength without increasing cross-sectional properties, suggesting that mechanical stimulation led to changes in the bone matrix, and these changes led to the improvements in mechanical properties. Consistent with this interpretation, the mineral/matrix ratio was significantly increased in exercised bones. The number of fatigue-induced microcracks was significantly lower in exercised bones, providing evidence that exercise modulates fatigue resistance. The ratio of nonreducible/reducible cross-links mirrored the damage data. Similar trends (exercise induced increases in mechanical properties without increases in cross-sectional properties, but with compositional changes) were also observed in 2-month-old biglycan-deficient and wild-type mice bred on a C57BL/6x129 genetic background.