Author(s): OGA, R., FURUMI, T., TAMAKI, H., Institution: NATIONAL INSTITUTE OF FITNESS AND SPORTS IN KANOYA, Country: JAPAN, Abstract-ID: 1173

INTRODUCTION:
Mechanical loading is a primary regulator of bone microarchitecture, and loading frequency modulates osteogenic responses by altering the temporal pattern of mechanical stress. However, loading frequency is often varied together with other mechanical parameters, such as loading magnitude or cycles, which can complicate the interpretation of frequency-specific effects. Therefore, this study compared the effects of 10 Hz and 40 Hz external mechanical loading on tibial bone microarchitecture using a cycle-matched and peak load–matched protocol in rats.
METHODS:
Eighteen 8-week-old male Fischer 344 rats were assigned to a control group (CON), a 10 Hz loading group (LF), or a 40 Hz loading group (HF) (n = 6/group). External mechanical loading (10 N, 1200 cycles) was applied to the left tibia along its longitudinal axis in a compressive manner twice weekly for 8 weeks. Following the intervention, at 16 weeks of age, in vivo contractile properties (twitch and maximal isometric force) of the tibialis anterior (TA) muscle were measured. Subsequently, the TA muscle and tibia were harvested. Muscle wet weight was recorded, and the TA was processed for histological analysis using hematoxylin and eosin staining to determine fiber cross-sectional area (fCSA). Trabecular and cortical bone microarchitecture were assessed by micro-computed tomography in the secondary spongiosa and mid-diaphyseal regions, respectively.
RESULTS:
Trabecular bone volume fraction in the LF group showed a trend to be higher than in CON, although this difference was not statistically significant. The LF group also exhibited significantly lower trabecular bone separation than the CON group (p = 0.043), accompanied by trends toward higher trabecular number and connectivity density. In contrast, no significant differences in these trabecular parameters were observed between the HF and CON groups or between the LF and HF groups. Cortical bone parameters remained unchanged across all groups. Regarding skeletal muscle outcomes, muscle wet weight, fCSA, and maximal contractile force did not differ among groups.
CONCLUSION:
Low-frequency external mechanical loading at 10 Hz is sufficient to induce trabecular bone adaptation, whereas increasing loading frequency to 40 Hz does not confer additional bone structural benefit when loading cycles and peak load are matched. Importantly, these bone adaptations occurred without concurrent changes in skeletal muscle mass or contractile function, supporting a bone-specific response to externally applied mechanical loading.