People
walking normally, women tottering in high heels and ostriches strutting
all exert the same forces on the ground despite very differently-shaped
feet, according to research funded by the Wellcome Trust and the
Biotechnology and Biological Sciences Research Council. The finding
suggests that prosthetic lower limbs and robots’ legs could be made more
efficient by making them less human-like and more like the prosthetics
used by ‘Blade Runner’ Oscar Pistorius.
Walking
involves a repeated process referred to by scientists as ‘crash, vault,
push’ – landing (‘crashing’) on the heel, vaulting over the stationary
leg and then pushing off with the toes. This is the most economical way
of walking and, as research published today in the Journal of the Royal
Society Interface shows, the force exerted on the ground is the same for
people walking normally or in high heels and for ostriches.
Dr
Tatjana Hubel from the Royal Veterinary College explains: “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. This is the most mechanically
economical way of walking. We do everything we can to make the forces
follow the same pattern, which is why for example women wiggle their
bottoms 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?”
When
scientists model how the leg moves, they tend to simplify the movement
and view the leg as a stick with a block on top (the body) which moves
in an inverted pendulum motion. In this simplified model, the shape of
the human foot does not make sense. But in fact, the human leg is more
complicated than this; it contains muscles that likely evolved out of a
tension between being optimised for walking and being more efficient at
running. As humans are intelligent and able to plan and use tools, being
able to move quickly to catch a prey or to evade a predator is not
essential.
The
shape of the human foot means that when the foot is flat on the ground,
all the force goes through the ankles, allowing the muscles to support
the weight of the body whilst being largely unloaded during the ‘vault’
stage. When muscles bear a load, they get tired easily, even if they are
doing no work. For example, if we hold our arms outstretched, after a
few minutes they will grow tired; by comparison, a JCB digger can extend
its arm indefinitely.
The
researchers believe this finding may have implications for the design
of better prosthetic limbs for above-knee amputees and for the legs of
humanoid robots. These might be improved by bearing more resemblance to
an ostrich leg than that of a human.
Dr
Jim Usherwood, a Wellcome Trust Senior Research Fellow at the Royal
Veterinary College, explains: “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. There’s no point in putting the motor at the end of the foot,
where it makes the leg more difficult to swing forwards—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.
An
example of this kind of prosthetic already in use are the blades used
by Paralympic athlete Oscar Pistorius—the ‘Blade Runner’. These blades
are light, springy and without a heel, similar to an ostrich’s legs,
which are optimised for running from predators rather than for walking.
Source: Wellcome Trust
