Prosthetics could be designed to be more efficient if they are made less like human feet and more like those of a woman wearing high heels or an ostrich, according to a study recently published in the Journal of the Royal Society Interface.
The researchers found that despite the differences in foot shape, the ground force reactions exerted by people walking normally, women in high heels and ostriches were the same.
“Despite vastly differing arrangements of joints and hip wiggles, humans walking normally, women in extremely high heels and ostriches all produce strikingly similar forces when walking,” Tatjana Hubel, PhD, from the Royal Veterinary College, stated in a press release. “We do everything we can to make the forces follow the same pattern, which is why for example women wiggle when they’re in high heels. The question for us is, why is the human foot shaped the way that it is and not, say, like an ostrich’s?”
Human walking involves a repeated process referred to as crash, vault, push, during which the walker lands on the heel, vaults over the stationary leg and pushes off with the toes. This is the most economical way of walking based on the design of the human foot, which has evolved as a result of advancements in technology that no longer require humans to quickly transition from walking to running in order to catch prey or escape predators. Due to the modern shape of the foot, while the foot is flat on the ground, all of the force is transmitted through the ankles, which could cause the muscles to tire easily.
The study results suggest that prosthetic legs for above-knee amputees could be more efficient if they were modeled after the leg of an ostrich, rather than a human leg.
“If you want to make a good prosthetic foot but don’t care what it looks like, you should put the motor—in this case, the ankle—as far up the leg as possible, where it can provide the power without making the feet heavy and hard to swing backwards and forwards,” Jim Usherwood, PhD, lead author and a Wellcome Trust senior research fellow at the Royal Veterinary College, stated in the release. “There’s no point in putting the motor at the end of the foot, where it makes the leg more difficult to swing forwards, which is important in both walking and running.
“Some clever prosthetics copy the ankle and are very human-like, which is fine for prosthetics to replace the foot, but for above-knee amputee, a typical prosthetic leg which is very human-like is heavy and hard to move around. It’s much better to have an ostrich foot at the end of a very lightweight leg,” Usherwood added.
An example of this kind of prosthetic is the type used by Paralympic sprinter Oscar Pistorius, which is designed without a heel, like an ostrich, and optimized for running.