Comparison of human and prosthetic forefoot stiffness

The purpose of this study was to quantify human metatarsophalangeal joint stiffness and compare it to an existing artificial foot which is a candidate for the foot in a shoe testing robot. The study was conducted by examining the forefoot flexion produced by external moments about a Z axis, defined by the horizontal line segment between the 1^st and 5 ^th metatarsal heads, a vertical Y axis, and an X axis normal to each.

It was found that the largest stiffness coefficient was about the Z axis. An MTS machine was used to measure shoe stiffness. The stiffness of the shoe about the Z axis was relatively small in comparison to the human foot stiffness. These results showed that shoe stiffness does not dominate that of the foot about the Z axis and that foot stiffness must be incorporated into the artificial foot.

The artificial foot chosen was the Seattle Foot by Seattle Limb Systems Inc. The stiffness of the Seattle Foot was similarly determined by the MTS machine and was compared with the stiffness of the human MP joint. The stiffness of the prosthetic about the Z axis was found to be 37% of the average human stiffness. It was also found that while a human joint rotates about a point, this particular prosthetic foot bends as a beam throughout the forefoot. Thus, a modification to the foot was proposed in which stiff plates would prevent bending along a selected portion of the forefoot, thus increasing the net stiffness of the forefoot. The required dimensions of the plates were calculated based on a theoretical model, and upon testing the stiffness of the modified foot experimentally, it was found that the stiffness had increased to 94% of the desired stiffness, which was within half a standard deviation. (Abstract shortened by UMI.)