The energetic cost of adaptive feet in walking

While humanoid feet are made of rigid plates, human feet have evolved into highly articulated and flexible elements. This adaptiveness provides key advantages. It absorbs impacts and secures grip when interacting with the environment. However, the human foot design potentially increases the energetic cost, because it features actuators and provides less power transfer than a rigid plate does. Here we use neuromuscular models with different foot designs and show that human feet incur about 20% more energetic cost than rigid ones for walking speeds up to 1.2ms^-1, which is close to the preferred walking speed. Above this speed, human feet do not show an energetic disadvantage. In addition we propose a foot design for prosthetic or humanoid feet which preserves key features of adaptive feet but does not require actuation, and show that it reduces the energetic cost by 15% or more independent of the walking speed. We conclude that human evolution may have traded the advantages of adaptive feet for energy efficiency, and that robotic systems could gain the former without compromising on the latter.