Human bone becomes increasingly brittle with ageing. Bones also fracture differently under slow and fast loadings, being ductile and brittle, respectively. The effects of a combination of these two factors have never been examined before. Here we show that cortical bone is most fracture-resistant at the physiologically prevalent intermediate strain rates of 10-3 s-1 to 10-2 s-1 such as they occur in walking or running, slightly weaker at slower quasistatic and much weaker at fast impact loading rates. In young cortical bone (15 years of age) the ductile-to-brittle transition (DBT) occurs at strain rates of 10-2 s-1, in old cortical bone (85 yrs) at speeds lower by a factor of 10 to 40. Other research has shown that the energy required to break bone (per unit of fracture surface) drops as much as 60% between these two ages. Therefore, DBT seems to compound the well-known phenomenon of 'brittle old bones'. Old bones can only cope with slow movement, young ones with both slow and fast movement. These observed material characteristics of (i) a shift of the DBT and (ii) a reduced energy absorption capacity appear to contribute at least as much to the loss of bone quality as the various quantity based (lowered bone density and mineral content) explanations of the past. They also provide a new powerful paradigm, which allows us to demonstrate mechanically, and uniquely, how human bone becomes increasingly brittle with age.

中文翻译：

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老化骨折：随年龄变化的韧性向脆性转变的案例

随着年龄的增长，人体骨骼变得越来越脆弱。骨骼在慢速和快速加载下也会发生不同的断裂，分别是延展性和脆性。以前从未研究过这两个因素组合的影响。在这里，我们表明皮质骨在 10-3 s-1 到 10-2 s-1 的生理普遍中间应变率下最耐骨折，例如它们发生在步行或跑步时，在较慢的准静态时稍弱，在冲击载荷速度快。在年轻的皮质骨（15 岁）中，韧性到脆性的转变 (DBT) 发生在 10-2 s-1 的应变率下，在旧的皮质骨（85 岁）中以 10 到 40 倍的速度发生其他研究表明，在这两个年龄段之间，骨折所需的能量（每单位骨折面）下降了 60%。所以，DBT 似乎加剧了众所周知的“脆骨”现象。老骨头只能应付慢动作，年轻的骨头可以应付慢动作和快动作。(i) DBT 的变化和 (ii) 能量吸收能力降低的这些观察到的材料特性似乎对骨质量损失的贡献至少与基于不同数量（降低的骨密度和矿物质含量）的解释一样多过去。它们还提供了一个新的强大范例，使我们能够机械地、独特地展示人类骨骼如何随着年龄的增长而变得越来越脆弱。(i) DBT 的变化和 (ii) 能量吸收能力降低的这些观察到的材料特性似乎对骨质量损失的贡献至少与基于不同数量（降低的骨密度和矿物质含量）的解释一样多过去。它们还提供了一个新的强大范例，使我们能够机械地、独特地展示人类骨骼如何随着年龄的增长而变得越来越脆弱。(i) DBT 的变化和 (ii) 能量吸收能力降低的这些观察到的材料特性似乎对骨质量损失的贡献至少与基于不同数量（降低的骨密度和矿物质含量）的解释一样多过去。它们还提供了一个新的强大范例，使我们能够机械地、独特地展示人类骨骼如何随着年龄的增长而变得越来越脆弱。