Ishida Akimasa, Masuda Tadashi, Inaoka Hidenori, Fukuoka Yutaka
Institute of Biomaterials and Biomedical Engineering, Tokyo Medical and Dental University, Tokyo, Japan.
Med Biol Eng Comput. 2008 Mar;46(3):213-21. doi: 10.1007/s11517-007-0269-8. Epub 2007 Oct 10.
During an upright stance of humans, it is usually assumed that a stiffer ankle joint contributes to stabilize the stance. To show that under certain conditions a stiffer ankle joint can reduce the stability, the frequency responses of the moment and the angle of the ankle joint against external disturbances caused by random horizontal translations of the support surface were evaluated in ten healthy adult subjects by varying the difficulty of the task at four levels. When it was difficult to keep the upright stance, the subject tended to make the ankle joint stiffer. The transfer function relating the external disturbance moment to the ankle joint moment showed a larger gain in the high frequency range (>0.3 Hz) compared with the gains obtained under easier conditions. A simulation analysis based on a simple inverted pendulum model also reproduced this tendency. These results indicate that the stiffer ankle joint and the resulting higher ankle moment for high frequency external disturbances enhance the possibility that the center of pressure exceeds the limit arising from the size of the feet and can make the upright stance unstable.
在人类直立站立时，通常认为踝关节更僵硬有助于稳定站立姿势。为了表明在某些情况下，更僵硬的踝关节会降低稳定性，通过在四个水平上改变任务难度，对十名健康成年受试者在支撑面随机水平平移引起的外部干扰下，踝关节力矩和角度的频率响应进行了评估。当难以保持直立姿势时，受试者倾向于使踝关节更僵硬。与在较容易条件下获得的增益相比，将外部干扰力矩与踝关节力矩相关联的传递函数在高频范围（>0.3Hz）显示出更大的增益。基于简单倒立摆模型的模拟分析也重现了这种趋势。这些结果表明，对于高频外部干扰，更僵硬的踝关节以及由此产生的更高踝关节力矩增加了压力中心超过由脚的大小所产生的极限的可能性，并可能使直立姿势不稳定。
