Short-term moderate-intensity treadmill running may confer a greater benefit to bone strength than high-intensity treadmill running in 12-month-old C57BL/6 mice

Ageing is linked to pathological changes in bone structure and the loss of bone mass and strength. Exercise is a non-pharmacological intervention that may improve bone mass; however, the effects on bone strength, structure, and material properties remain unclear. We tested the effects of work-matched moderate- and high-intensity treadmill exercise on bone structure and strength in the mature (middle-aged) murine skeleton. Twelve-month-old male mice (considered middle-aged in C57BL/6 strain) underwent high-intensity interval training (HIIT, 4 × 4 min, 85-90 % maximum speed) or moderate-intensity continuous training (MICT, 24 min, 60 % maximum speed) three times per week for six weeks. Trabecular and cortical tibial bone microarchitecture were assessed using micro-computed tomography and compared to an age-matched, sedentary cohort and a 12-week-old adult cohort. The effects of ageing were evident in both trabecular and cortical bone, characterised by lower trabecular bone mass, lower cortical area, and thinner yet denser cortices in 12-month-old mice compared to the younger group. Three-point bending tests of the bone, corrected for bone size, revealed that the HIIT tibiae exhibited lower ultimate stress, yield stress, and elastic modulus than the MICT group. While neither HIIT nor MICT significantly differed from sedentary controls, this suggests that moderate-intensity treadmill running in 12-month-old mice may provide greater mechanical protection to the skeleton than high-intensity treadmill running. No significant differences were detected in trabecular or cortical bone mass or structure between exercised and sedentary groups, apart from trabecular separation, which was lower in the HIIT group, suggesting a mild benefit. Future studies should explore whether extended training and/or incorporating resistance exercise into these interventions could increase bone strength in older mice.