BACKGROUND: Bone morphological stereological observation is limited due to the difficulty in human experimental bone access. Animal studies overcome the limitation of human experiments and can directly evaluate bone quality through bone stereological histomorphometric parameters.OBJECTIVE: To explore the mechanism of the impacts of exercise on long bone development in adolescents through exercise and load experiments in rats.DESIGN: A randomized controlled study by employing experimental animals as subjects.SETTING: Laboratory of Sports of Beijing University of Physical Education and the Key Laboratory of Orthopaedics of Xijing Hospital Affiliated to the Fourth Military Medical University of Chinese PLA.MATERIALS: The study was conducted in the Laboratory of Sports of Beijing University of Physical Education and the Key Laboratory of Orthopaedics of Xijing Hospital Affiliated to the Fourth Military Medical University of Chinese PLA. Totally 120 male 4-week old SD rats were randomly paired into 12 groups with 10 rats each based on their body mass.METHODS: Incremental exercises with the combination of suspension and loading mode of rats were used in the study. The undecalcified slices of the right tibia were observed after double-fluorescence-labeling by tetracycline (TC) and calcein for the analysis of bone histomorphometric parameters including bone mass, structure and metabolism, etc. During the period of growth under different exercise strength and different modes of mechanic load by stereological sampling without bias.static or dynamic parameters of bone trabecula among groups of different modes for mechanical load.RESULTS: The increase or decrease of multiple parameters including bone mass, bone quality, bone structure and bone metabolism in low-loading exercise group were more regular than that of high-loading exercise group and control group. In the suspension group, the bone mineral apposition significantly reduced, and the relative volume of bone trabecula significantly decreased with the significantly increased thickness, decreased numbers and significant of increased separation. The biochemical indicators of bone metabolism and the biomechanical capability of suspension group were significantly lower than that of low-loading exercise group but similar to that of control group. All parameters of loading group were similar to those of low-loading exercise group with the characters of active osteogenesis and osteo-absorption but mainly osteogenesis, and significantly enhanced abilities of anti-destruction and anti-deformity in bone.CONCLUSION: Exercises of different strengths can produce different impacts on long bone development in rats of growth period. The integrated effects of low-loading exercises in bone metabolism, bone mass, bone histomorphology and bone biomechanics are better than those of high-loading exercises. This phenomenon suggests that appropriate increment of gravity load during the exercise processin children and adolescents is good for the improvement of the capability of bone metabolism and bone mechanics, which further improve the bone quality during development.