Treadmill Exercise Improves Fracture Toughness and Indentation Modulus without Altering the Nanoscale Morphology of Collagen in Mice

The specifics of how the nanoscale properties of collagen (e.g., the crosslinking profile) affect the mechanical integrity of bone at larger length scales is poorly understood despite growing evidence that collagen's nanoscale properties are altered with disease. Additionally, mass independent increases in postyield displacement due to exercise suggest loadinginduced improvements in bone quality associated with collagen. To test whether diseaseinduced reductions in bone quality driven by alterations in collagen can be rescued or prevented via exercise-mediated changes to collagen's nanoscale morphology and mechanical properties, the effects of treadmill exercise and beta-aminopropionitrile treatment were investigated. Eight week old female C57BL/6 mice were given a daily subcutaneous injection of either 164 mg/kg beta-aminopropionitrile or phosphate buffered saline while experiencing either normal cage activity or 30 min of treadmill exercise for 21 consecutive days. Despite differences in D-spacing distribution (P = 0.003) and increased cortical area (tibial: P = 0.005 and femoral: P = 0.015) due to beta-aminopropionitrile treatment, an overt mechanical disease state was not achieved as there were no differences in fracture toughness or 4 point bending due to beta-aminopropionitrile treatment. While exercise did not alter (P = 0.058) the D-spacing distribution of collagen or prevent (P < 0.001) the beta-aminopropionitrile-induced changes present in the unexercised animals, there were differential effects in the distribution of the reduced elastic modulus due to exercise between control and beta-aminopropionitrile-treated animals (P < 0.001). Fracture toughness was increased (P = 0.043) as a main effect of exercise, but no significant differences due to exercise were observed using 4 point bending. Future studies should examine the potential for sex specific differences in the dose of beta-aminopropionitrile required to induce mechanical effects in mice and the contributions of other nanoscale aspects of bone (e.g., the mineral - collagen interface) to elucidate the mechanism for the exercise-based improvements in fracture toughness observed here and the increased postyield deformation observed in other studies.