Walking involves a repeated process referred to by scientists as'crash, vault, push' -- landing ('crashing') on the heel, vaultingover the stationary leg and then pushing off with the toes. This isthe most economical way of walking and, as research published May 8in the Journal of the Royal Society Interface shows, the force exerted on the ground is the same for peoplewalking normally or in high heels and for ostriches. Dr Tatjana Hubel from the Royal Veterinary College explains:"Despite vastly differing arrangements of joints and hipwiggles, humans walking normally, women in extremely high heels andostriches all produce strikingly similar forces when walking. Thisis the most mechanically economical way of walking. "We do everything we can to make the forces follow the samepattern, which is why -- for example -- women wiggle their bottomswhen they are in high heels. |
The question for us is, why is thehuman foot shaped the way that it is and not, say, like anostrich's?" When scientists model how the leg moves, they tend to simplify themovement and view the leg as a stick with a block on top (thebody), which moves in an inverted pendulum motion. In thissimplified model, the shape of the human foot does not make sense. In reality, however, the human leg is more complicated than this;it contains muscles that probably evolved out of a tension betweenbeing optimised for walking and being more efficient at running.Because humans are intelligent and can plan and use tools, beingable to move quickly to catch prey or evade a predator is notessential. The shape of the human foot means that when the foot is flat on theground, all the force goes through the ankles, allowing the musclesto support the weight of the body while being largely unloadedduring the 'vault' stage.
When muscles bear a load, they get tiredeasily, even if they are doing no work. For example, if we hold ourarms outstretched, after a few minutes they will grow tired; bycomparison, a JCB digger can extend its arm indefinitely. The researchers believe this finding might have implications forthe design of better prosthetic limbs for above-knee amputees andfor the legs of humanoid robots. These might be improved by bearingmore resemblance to an ostrich leg than that of a human. Dr Jim Usherwood, a Wellcome Trust Senior Research Fellow at theRoyal Veterinary College, explains: "If you want to make agood prosthetic foot but don't care what it looks like, you shouldput the motor -- in this case, the ankle -- as far up the leg aspossible, where it can provide the power without making the feetheavy and hard to swing backwards and forwards.
There's no point inputting the motor at the end of the foot, where it makes the legmore difficult to swing forwards -- important in both walking andrunning. "Some clever prosthetics copy the ankle and are veryhuman-like, which is fine for prosthetics to replace the foot, butfor above-knee amputees, a typical prosthetic leg that is veryhuman-like is heavy and hard to move around. It's much better tohave an ostrich foot at the end of a very lightweight leg." One example of this kind of prosthetic already in use is the bladesused by Paralympic athlete Oscar Pistorius -- the 'Blade Runner'.These blades are light, springy and without a heel, similar to anostrich's legs, which are optimised for running from predatorsrather than for walking.
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