Landing Produced Different Adaptation Behaviors On Bone Biomaterial Between Fore-limb And Hind-limb In Rats

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Mechanical loading induced tissue strain has been mentioned as a critical factor to stimulate more bone formation. Typical jumping movement includes two phases, landing and take off. Landing phase generally produced a higher ground reaction force (GRF) and subsequently a higher strain rate would generate on bone tissue. PURPOSE: The purpose of this study was to investigate the effects of a short-term free-fall landing training on biomechanical properties of ulnae and femora in growing female rats. METHODS: Thirty-two female Wistar rats (7-week-old) were randomly assigned into three groups, which were the L30 (n=11), L10 (n=11) and CON (n=10) groups. Animals of the L30 and L10 groups respectively subjected to 30 and 10 free-fall landings per day from a height of 40 cm for five days. Landing GRF was recorded on day 1 and day 5. Biomechanical properties measurements were performed on animals' ulnae and femora. One-way ANOVA was used for statistical analysis. When statistical significance was shown (p<0.05), pair-wise comparisons were made using the Fisher's LSD method. RESULTS: In GRF, significant differences were shown between hind-limbs and fore-limbs. In biomechanical test, ulnae of two landing groups showed significantly higher post-yield energy (mJ) (L10: 23.7±2.1, L30: 25.8±1.4 VS. CON: 18.7±1.2, p<0.05) and marginally fracture energy (mJ) as compared to the CON group (L10: 36.1±2.3, L30: 34.3±2.5 VS. CON: 28.8±2.0, p=0.073). In femora, however, post-yield energy (mJ) (L10: 27.7±3.97, L30: 27.0±4.10 VS. CON: 42.5±5.89, p<0.05) and ultimate toughness (mJ/mm3) was significantly higher in the CON group as compared to the L10 or L30 group (L10: 3.1±0.1, L30: 3.3±0.1 VS. CON: 4.2±0.4, p<0.05). CONCLUSIONS: Bone adapting to mechanical loading is site-specific and timing dependent. Landing induced bone matrix reorganization (e.g. integration between cross-links and collagen) and its contribution to tissue biomaterial properties are valuable for further research.