Abstract: BACKGROUND: Postmenopausal osteoporosis significantly increases the risk of fracture, which seriously affects the quality of life of patients. Exercise therapy is an important non-drug means and prevention and treatment strategy for patients with osteoporosis, in which strength training is the best mode, but its specific biological mechanism has not been determined. 
OBJECTIVE: To investigate the effects of strength training on bone morphology, materials and biomechanics in ovariectomized rats and to explore the mechanism of extracellular matrix remodeling. 
METHODS: Forty-eight female Sprague-Dawley rats were divided into sham operation group, sham operation exercise group, ovariectomized group and ovariectomized exercise group according to the random number table method. The menopausal animal model was established by bilateral ovariectomy in the ovariectomized group and ovariectomized exercise group, while sham operation was performed in the sham operation group and sham operation exercise group. Four weeks after operation, the sham operation exercise group and the ovariectomized exercise group underwent 12-week tail weight-bearing ladder training, and the sham operation group and the ovariectomized group were raised quietly in the cage. The bilateral femur and tibia were separated after training. The right tibia was used for dual-energy X-ray densitometry and biomechanical, biophysical and biochemical analyses, the left tibia was detected using micro-computed tomography for bone microstructural examination, the right femur was subjected to hematoxylin-eosin staining for histological observation, and the left femur was used for western blot and gelatin zymography detection of protein expression and enzyme activity of extracellular matrix metabolism-related factors, respectively. 
RESULTS AND CONCLUSION: Compared with the sham operation group, the maximal load and stiffness decreased (P < 0.05), bone density, bone mineral density, bone inorganic matter content, bone calcium content decreased (P < 0.05), bone water content increased (P < 0.05), trabecular bone volume fraction, trabecular connectivity density, and trabecular number decreased (P < 0.05), trabecular separation, structural model index increased (P < 0.05), bone adipocyte number and cross-sectional area increased (P < 0.05), matrix metalloproteinase-2 activity decreased (P < 0.05), and protein expression of tissue inhibitor of metalloproteinase-1 and osteoprotegerin increased (P < 0.05) in the ovariectomized group. Compared with the ovariectomized group, the maximal load, stiffness, fracture load and resilience increased (P < 0.05), bone mineral density, bone mineral content, bone mineral density, bone inorganic matter content, and bone calcium content increased (P < 0.05), bone water content decreased (P < 0.05), trabecular separation and bone marrow area decreased (P < 0.05), trabecular bone thickness, cortical bone volume fraction, cortical bone area fraction, cortical bone thickness, and cortical bone porosity increased (P < 0.05), bone adipocyte number and cross-sectional area reduced (P < 0.05), matrix metalloproteinase-2 activity increased (P < 0.05), and protein expression of tissue inhibitor of metalloproteinase-1, Runt-related transcription factor 2 and osteoprotegerin decreased (P < 0.05) in the ovariectomized exercise group. To conclude, strength training can protect against bone injury caused by estrogen deficiency, which is characterized by improvement of bone biomechanical properties, bone tissue composition and bone microstructure, and its mechanism is related to the regulation of extracellular matrix remodeling.

Key words: strength training, ovariectomy, menopause, osteoporosis, biomechanics, extracellular matrix