Intelligent Prosthetic Systems, LLC, Ann Arbor, MI 48104, USA.
J Exp Biol. 2013 Jul 15;216(Pt 14):2722-31. doi: 10.1242/jeb.082347. Epub 2013 Apr 11.
During human walking, the center of pressure under the foot progresses forward smoothly during each step, creating a wheel-like motion between the leg and the ground. This rolling motion might appear to aid walking economy, but the mechanisms that may lead to such a benefit are unclear, as the leg is not literally a wheel. We propose that there is indeed a benefit, but less from rolling than from smoother transitions between pendulum-like stance legs. The velocity of the body center of mass (COM) must be redirected in that transition, and a longer foot reduces the work required for the redirection. Here we develop a dynamic walking model that predicts different effects from altering foot length as opposed to foot radius, and test it by attaching rigid, arc-like foot bottoms to humans walking with fixed ankles. The model suggests that smooth rolling is relatively insensitive to arc radius, whereas work for the step-to-step transition decreases approximately quadratically with foot length. We measured the separate effects of arc-foot length and radius on COM velocity fluctuations, work performed by the legs and metabolic cost. Experimental data (N=8) show that foot length indeed has much greater effect on both the mechanical work of the step-to-step transition (23% variation, P=0.04) and the overall energetic cost of walking (6%, P=0.03) than foot radius (no significant effect, P>0.05). We found the minimum metabolic energy cost for an arc foot length of approximately 29% of leg length, roughly comparable to human foot length. Our results suggest that the foot's apparently wheel-like action derives less benefit from rolling per se than from reduced work to redirect the body COM.
在人类行走过程中，脚底下的压力中心在每一步中平稳地向前推进，在腿部和地面之间形成一种类似轮子的运动。这种滚动运动似乎有助于行走的经济性，但导致这种好处的机制尚不清楚，因为腿部并不是真正的轮子。我们提出，确实存在好处，但与其说是来自滚动，不如说是来自类似钟摆的支撑腿之间更平滑的过渡。在这种过渡中，身体质心（COM）的速度必须被重新定向，而更长的脚会减少重新定向所需的功。在这里，我们开发了一个动态行走模型，预测了改变脚长而不是脚半径的不同效果，并通过将刚性的、弧形的脚底附着在固定脚踝的人类身上进行了测试。该模型表明，平滑滚动对弧形半径的相对不敏感，而步与步之间的过渡功则随脚长近似二次减少。我们分别测量了弧形脚长度和半径对 COM 速度波动、腿部做功和代谢成本的影响。实验数据（N=8）表明，脚长对步与步之间的过渡的机械功（23%的变化，P=0.04）和行走的整体能量成本（6%，P=0.03）都有更大的影响，而脚半径则没有显著影响（P>0.05）。我们发现弧形脚长度大约为腿长的 29%时，代谢能量成本最低，大约相当于人类的脚长。我们的结果表明，脚部明显的类似轮子的动作，其本身从滚动中获得的好处不如从重新定向身体 COM 中减少的功那么多。
