The cost of being on your toes

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The cost of being on your toes

Walking heels-first is less work than walking on your toes or balls of the feet

(images at http://www.eurekalert.org/pub_releases/2010-02/uou-tco020310.php)

Humans, other great apes and bears are among the few animals that step first on

the heel when walking, and then roll onto the ball of the foot and toes. Now, a

University of Utah study shows the advantage: Compared with heel-first walking,

it takes 53 percent more energy to walk on the balls of your feet, and 83

percent more energy to walk on your toes.

" Our heel touches the ground at the start of each step. In most mammals, the

heel remains elevated during walking and running, " says biology Professor

r, senior author of the new study being published online Friday, Feb. 12

and in the March 1 print issue of The Journal of Experimental Biology.

" Most mammals – dogs, cats, raccoons – walk and run around on the balls of their

feet. Ungulates like horses and deer run and walk on their tiptoes, " he adds.

" Few species land on their heel: bears and humans and other great apes – chimps,

gorillas, orangutans. "

" Our study shows that the heel-down posture increases the economy of walking but

not the economy of running, " says r. " You consume more energy when you

walk on the balls of your feet or your toes than when you walk heels first. "

Economical walking would have helped early human hunter-gatherers find food, he

says. Yet, because other great apes also are heel-first walkers, it means the

trait evolved before our common ancestors descended from the trees, he adds.

" We [human ancestors] had this foot posture when we were up in the trees, "

r says. " Heel-first walking was there in the great apes, but great apes

don't walk long distances. So economy of walking probably doesn't explain this

foot posture [and why it evolved], even though it helps us to walk

economically. "

r speculates that a heel-first foot posture " may be advantageous during

fighting by increasing stability and applying more torque to the ground to

twist, push and shove. And it increases agility in rapid turning maneuvers

during aggressive encounters. "

The study concludes: " Relative to other mammals, humans are economical walkers

but not economical runners. Given the great distances hunter-gatherers travel,

it is not surprising that humans retained a foot posture, inherited from our

more arboreal [tree-dwelling] great ape ancestors, that facilitates economical

walking. "

Measuring the Costs of Different Modes of Walking and Running

r conducted the study with Cunningham, a doctoral student in

biology at the University of Utah; Nadja Schilling, a zoologist at Friedrich

Schiller University of Jena, Germany; and Christoph Anders, a physician at

University Hospital Jena. The study was funded by the National Science

Foundation, Friedrich Schiller University of Jena and a German food industry

insurance group interested in back pain.

The study involved 27 volunteers, mostly athletes in their 20s, 30s and 40s.

Each subject walked or ran three different ways, with each step either

heel-first, ball-of-foot first with the heel a bit elevated or toes first with

the heel even more elevated.

In his lab, r and colleagues measured oxygen consumption – and thus energy

use – as 11 volunteers wore face masks while walking or running on a treadmill.

They also walked on a " force plate " to measure forces exerted on the ground.

Part of the study was conducted at Anders' lab in Germany, where 16 people

walked or ran on a treadmill as scientists monitored activity of muscles that

help the ankles, knees, hips and back do work during walking and running.

Findings of the experiments included:

" You consume more energy when you walk on the balls of your feet or your toes

than when you walk heels-first, " r says. Compared with heels-first

walkers, those stepping first on the balls of their feet used 53 percent more

energy, and those stepping toes-first expended 83 percent more energy.

" The activity of the major muscles of the ankle, knee, hip and back all increase

if you walk on the balls of your feet or your toes as opposed to landing on your

heels, " says r. " That tells us the muscles increase the amount of work

they are producing if you walk on the balls of your feet. "

" When we walk on the balls of our feet, we take shorter, more frequent strides, "

r says. " But this did not make walking less economical. " Putting the heel

down first and pivoting onto the ball of the foot makes the stride longer

because the full length of the foot is added to the length of the step. But that

has no effect on energy use.

The researchers wondered if stepping first on the balls of the feet took more

energy than walking heel-first because people are less stable on their toes or

balls of the feet. But increased stability did not explain why heel-first

walking uses less energy.

Stepping heel-first reduced the up-and-down motion of the body's center of mass

during walking and required less work by the hips, knees and ankles. Stepping

first onto the balls of the feet slows the body more and requires more

re-acceleration.

Heels-first steps also made walking more economical by increasing the transfer

of movement or " kinetic " energy to stored or " potential " energy and back again.

As a person starts to step forward and downward, stored energy is changed to

motion or kinetic energy. Then, as weight shifts onto the foot and the person

moved forward and upward, their speed slows down, so the kinetic energy of

motion is converted back into stored or potential energy. The study found that

stepping first onto the balls of the feet made this energy exchange less

efficient that walking heels-first.

Heel-first walking also reduced the " ground reaction force moment " at the ankle.

That means stepping first onto the ball of the foot " decreases the leverage,

decreases the mechanical advantage " compared with walking heel-first, r

says.

In sum, walking heel-first is not more economical because it is more stable or

involves fewer, longer strides, but because when we land on our heels, less

energy is lost to the ground, we have more leverage, and kinetic and potential

energy are converted more efficiently.

Form and Function of the Foot

If heel-first walking is so economical, why do so many animals walk other ways?

" They are adapted for running, " r says. " They've compromised their economy

of walking for the economy of running. "

" Humans are very good at running long distances. We are physiologically and

anatomically specialized for running long distances. But the anatomy of our feet

is not consistent with economical running. Think of all the animals that are the

best runners – gazelles, deer, horses, dogs – they all run on the ball of their

feet or the tips of their toes. "

When people run, why is there no difference in the amount of energy they expend

when stepping first onto their heels versus the balls of their feet or toes?

The answer is unknown, but " if you land on your heel when you run, the force

underneath the foot shoots very quickly to the ball of your foot, " r says.

" Even when we run with a heel plant, most of the step our weight is supported by

the ball of our foot. Lots of elite athletes, whether sprinters or distance

runners, don't land on their heel. Many of them run on the balls of their feet, "

as do people who run barefoot. That appears to be the natural ancestral

condition for early human runners, he adds.

" The important thing is we are remarkable economical walkers, " r says. " We

are not efficient runners. In fact, we consume more energy to run than the

typical mammal our size. But we are exceptionally economical walkers. "

" This study suggests that one of the things that may explain such economy is the

unusual structure of our foot, " he adds. " The whole foot contacts the ground

when we walk. We have a big heel. Our big toe is as long as our other toes and

is much more robust. Our big toe also is parallel to and right next to the

second toe. "

" These features are distinct among apes, and provide the mechanical basis for

economical walking. No other primate or mammal could fit into human shoes. "