Roger Hunter <[email hidden]> wrote:
Ozzie said:I'd be interested in what ways you'd describe what I'm attempting to describe regarding running
form and style.
Roger replied Bzzzzzz. May I interrupt? No you're not - interested, that it. The "tell me how you'd
put it" approach is a fairly standard ploy to draw individuals into an arena that they may not even
wish to enter. How can you offer standardised comments on form and style? 'Tis impossible. The human
body is a wonderful thing, but standardised it ain't. To even imply there is an 'idealised running
style' is to ignore the countless variations that exist amongst the human species. There *is* an
ideal running style for each of us. There *is not* an ideal running style.
Now continue...
Ozzie said:Especially with the differences I've seen between good runners with minimal vertical
displace/knee lift and runners with a lot more vertical displacement of their body than necessary
and no knee lift
Roger replied: "Good runners"???? "Runners with more vertical displacement....than necessary"????
Yes, I know what you're saying, but you take my point? Any style evaluation has to take into account
individual factors. Even in individuals of the same build, height, and weight there will be
differences in flexibility, muscle tone, aerobic capacity, etc, etc. You can't propose on the one
hand that we're experiments of one, and on the other that there is some mythical set of standards to
which we should all aspire. Let people run. If they do it with all the grace a poise of a charging
Rhinoceros, then so be it.
Ozzie said:If you want to see the displacement visually, turn away from some runners who are 20 to 50 feet
away. They can be running away from you or toward you. Look like your doing a hamstring stretch
with your legs apart and look at the runners upside down. That altered perspective gives a clear
picture of the vertical displacement of the runners especially if they don't lift their knees
(meaning they have to lift their entire center of gravity higher with each step).
Roger,
The articles which follow give some of the reasons and research findings why I disagree with what
you're saying: "There *is not* an ideal running style."
Here's some of the research that has reinforced my own observations and experiments-of-one regarding
running form and style. I'm sure a number of rec.runners will find it interesting food for thought.
In health and on the run, Ozzie Gontang Maintainer - rec.running FAQ Director, San Diego Marathon
Clinic, est. 1975
Mindful Running: mindfulness.commr.aspOpen ↗ faqs.orgrunning faqOpen ↗
No Skycaps Needed 208.245.156.153output.cfmOpen ↗ By Carl Zimmer
One of the most startling sights on a first trip to Africa is a common one: women carrying things on
their heads. Try to carry a suitcase on your head, and you'll probably bite your tongue in
concentration and wave your arms madly for balance. But African women walk for miles with heavy jugs
of water or pots of food as if they weren't carrying anything. Energetically speaking, they aren't:
researchers have found that the women can carry enormous loads without using any extra energy. They
aren't defying any laws of physics, though; they're being good pendulums.
In 1977 a team of Harvard physiologists, in Kenya to study the locomotion of wild animals, found
themselves distracted by the load- carrying women. "When we tried to pick up the loads, they were
just amazing," says Norman Heglund. "We wondered how the devil they did it." As a first experiment,
Heglund's team convinced some Kenyan women to wear breathing masks as they carried their loads; the
idea was to measure the women's oxygen consumption, calculate how many calories they were burning,
and then compare their performance with that of non-Africans. There the experiment ran into a snag:
the non-African researchers couldn't match the carrying capacity of the Kenyan women, at least not
with their heads. Heglund and his colleagues had to resort to backpacks and to using old
measurements from American Army recruits.
Still, the results were extraordinary. The African women could carry a fifth of their weight
without burning a single extra calorie; and although larger loads did require more energy, the
increase was only half of that needed by the American soldiers. Some women could carry 70 percent
of their weight.
Funding agencies haven't exactly been desperate for the answer to this riddle, so it's only recently
that Heglund has managed to get a step closer to one. While spending a year teaching at the
University of Nairobi in 1989, he had some Kenyan women walk across force plates; last year in
Belgium he repeated the experiment with European students. Force plates are devices that register
the vertical and horizontal forces exerted by a walking animal.
A walking human is like a pendulum swinging. When the pendulum is at its lowest point, it is moving
fastest, and its energy is almost all kinetic energy of motion. As the pendulum climbs up one side
of its arc and is slowed and finally stopped by gravity, that energy isn't all lost; most of it is
stored as potential energy and is converted back into kinetic energy when the pendulum starts to
fall again. But some of the energy is lost to friction, both in the bearing and between the pendulum
and the air.
Similarly, when you walk, the kinetic energy of your forward movement turns into potential energy as
you rise on one foot and is converted back into kinetic energy as you fall onto the other foot. But
with each footfall, only 65 percent of that kinetic energy is carried over into the next step; 35
percent is lost, mostly to internal friction in your leg. That 35 percent has to be made up by your
leg muscles, which convert food energy into kinetic energy.
The 35 percent rule applies to Kenyan women too--until they start carrying things on their heads.
Heglund's force plate readings allowed him to calculate how much energy his subjects were
transferring from one step to the next. Without a load, Kenyan women and Europeans both transferred
65 percent. When the Europeans carried loads on their backs, they still lost 35 percent--but now,
since they were bearing more weight at the same speed, that 35 percent represented more energy in
absolute terms, which they made up by burning more calories. In contrast, the Africans simply became
better pendulums. When they carried a fifth of their body weight on their heads, they somehow
managed to transfer 75 percent of their energy from one step to the next, losing only 25 percent to
friction. With a greater load, one woman reduced her loss to 15 percent. Heglund doesn't know what
biomechanical trick the Kenyan women are using--they couldn't tell him--but it must have something
to do with carrying things on your head. People who carry things for a living, he notes, from
Kenyans to Sherpas, tend to use their heads. "It's just amateurs like us that use suitcases and
backpacks," Heglund says.
See How They Run Carl Zimmer 208.245.156.153output.cfmOpen ↗
Two legs are a lot to move, and four legs are considerable. But six legs are a quandary, and
forty-four formidable.
Here, according to Nilesh Patel, is how you paint the legs of an ant: Begin by finding the right
paint. Don't use Wite-Out (it flakes off), and don't use oil-based paints (they're so thick that
they make ants walk as if they're wearing casts). Acrylic is best, says Patel. And be sure to get
the thinnest, finest brush your local art supply store has to offer.
Next, chill the ant well. Patel, a 23-year-old senior at the University of California at Berkeley,
takes his ants to the integrative biology department's temperature-controlled room, which he cools
to 40 degrees. Inevitably the cold-blooded ants become sleepy. To completely immobilize them, Patel
first tried trapping them under a staple pushed gently into Styrofoam, but they managed to wriggle
free. Now he Scotch- tapes three of their six legs to a table. Gently pulling the free legs straight
with a pair of tweezers, he dabs them with his brush. "The more practice you have, the better you
get at it," says Patel. "When I first started, it took me about seven minutes, but now I can get an
ant out in two."
Patel learned to paint ants at the bidding of postdoctoral researcher Rodger Kram, whose work was
spurred by a simple question: Do ants run?
The science of animal locomotion is full of such simple questions; getting the answers has always
been the difficult part. In 1872, for example, Leland Stanford, a railroad tycoon and the founder of
Stanford University, got into a heated debate as to whether all four legs of a horse leave the
ground when it trots. Legend has it that he even bet $25,000 that they did. In any case, he did
bankroll famed landscape photographer Eadweard Muybridge to find out. Muybridge had horses trot down
a path strung with threads connected to a row of cameras; when the horses snapped the threads, the
cameras snapped the pictures. It took Muybridge years to perfect a shutter fast enough and film
sensitive enough to capture the images (he also needed some time off to defend
himself--successfully-- against the charge that he had murdered his wife's lover), but in 1877 he
was finally able to give Stanford his answer: a series of pictures of a horse in motion, one of
which showed the animal seemingly levitating, with all its legs in the air.
Like Muybridge, Kram wanted to take a series of pictures of an animal in motion--albeit a much, much
smaller animal. And, like Muybridge, he had some technical considerations. Because even his
high-speed video cameras couldn't easily distinguish a moving ant's six legs, he decided that the
three facing the camera had to be painted white. Next, he and Patel built a narrow plastic chute for
the painted ants to dash through, coating the walls with liquid Teflon to keep the ants from
climbing up its sides. Patel then spent several weeks videotaping ants traveling down the chute. A
prod from a pair of forceps or a puff of air was enough to get an ant moving. Patel would reach over
to the camera and switch it on in time to tape the insect's movements.
One morning last fall, one ant gave Patel some trouble. "As soon as I touched him he took off," he
recalls, "and I wasn't fast enough turning on the camera." By simply turning the camera on before
prodding the ant, however, he succeeded in capturing the animal's movements on tape. Later, watching
the tape in slow motion, he and Kram saw something remarkable. In previous trials, the ants had used
their normal gait, known as the alternating tripod, in which two legs on one side and one leg on the
opposite side move in concert--the ant would raise, say, its middle left leg and its front and back
right legs, bring them forward, plant them on the ground, then repeat the movement with its other
three legs. But the tape of Patel's swift ant showed that at one point all its legs actually left
the ground. It was the first image of a running ant ever captured. "Unfortunately," says Kram,
"nobody had bet any money on it."
Kram and Patel, along with the dozen other Berkeley researchers who make up the PolyPEDAL lab (PEDAL
stands for Performance, Energetics, and Dynamics of Animal Locomotion), are modern Muybridges. But
instead of exposing the hidden complexities in the movement of tetrapods--animals like ourselves,
with four limbs--the PolyPEDAL lab specializes in the even more complicated mysteries of arthropods,
creatures that move on six or more legs and have external skeletons. Among the lab's menagerie, in
addition to the trotting ants, you'll find galloping crabs, undulating centipedes, and a cockroach
so fast it's made the Guinness Book of World Records.
"We don't study arthropods because we like them," says Robert Full, the 36-year-old physiologist who
directs the lab. "Many of them are actually disgusting. But they tell us secrets of nature that we
can't find out from studying one species, such as humans."
They are also telling the researchers secrets that may soon be applied to some very unnatural
creatures. For decades engineers and computer scientists have been convinced that walking,
insectlike robots would be ideal for moving over rough terrain. Thus far, though, the slow, awkward
machines they've built embody none of the speed and grace of insects. That, Full says, is because
they've based most of their designs on old assumptions about how insects move, assumptions the
PolyPEDAL lab has shown are false.
Until the PolyPEDAL lab came along, researchers looking at animal locomotion concentrated on
tetrapods even though there are several hundred times more arthropod species on this planet. It's
hard to blame the researchers, though. After all, it's much easier to film and analyze the four
heavy legs of a running dog than the six nearly weightless limbs of a skittering roach.
After completing his famous studies of horse movement, Muybridge went on to photograph many other
tetrapods, including humans, and showed that, as a rule, they all lifted their legs off the ground
simultaneously while running. That complete lack of contact with the ground, in fact, came to define
the act of running. It's a fairly crude definition, though--it tells you only about a brief moment
in an animal's gait and doesn't describe the rest of the motion. These days researchers are
constructing better definitions of walking and running by focusing on two simple models: pendulums
and pogo sticks.
A pendulum can swing for a long time because it continually recovers its energy. On its downward
stroke, it's powered by the force of gravity; when it reaches the lowest point of its arc it has so
much energy that it can counteract gravity and swing upward. When you walk, your body behaves like
an upside-down pendulum: the foot you plant in front of you is the pendulum's axis, your center of
mass the hanging weight. In the beginning of your stride you work against gravity, vaulting your
center of mass upward with your leg until you reach your highest point. Gravity then takes over, and
your body swings downward until your other leg hits the ground. The next stride is even easier. You
can use the energy given to you by gravity to vault yourself into your second and all successive
steps, just as a pendulum reclaims its energy in each swing.
When you run, however, you stop behaving so much like a pendulum and begin behaving more like a pogo
stick. Now when you first plant your leg, your body sinks down on it instead of rising up. Your leg
actually acts as a brake for your body, and so your center of mass is at its lowest point when your
acceleration is lowest. Meanwhile your tendons are acting as springs. As they stretch and snap back,
they store and release energy, just like the spring in a pogo stick, and propel you upward and
forward. Thus running, like walking, recovers energy and cuts down on your expenditure.
One intriguing implication of the pogo-stick model is that you don't have to have both feet in the
air to be running. In 1986 Thomas McMahon, a biomechanicist at Harvard, videotaped six runners
trying to imitate that miracle of locomotion, Groucho Marx. They deliberately bent their legs deeply
as they ran, so that they always kept one foot on the ground, yet their bodies still behaved like
pogo sticks. So much for Muybridge.
At the time, Full was working in the laboratory of McMahon's collaborator, [censored] Taylor,
investigating the intricacies of crab metabolism. One of the things he needed to know was what the
crabs were doing with their limbs: Were they using any kind of energy-saving gait? The conventional
wisdom at the time--which, as is often the case with conventional wisdom, had no real evidence to
back it up--was that if tetrapods were pendulums and pogo sticks, then arthropods were wheels. Like
a wheel, it was thought, an arthropod's center of mass moved forward at a constant speed, never
rising or falling, and so the animal never recovered energy as it moved.
To study crab locomotion, Full wanted to use one of the lab's force plates--a device that measures
the impact of an animal's footsteps. Generally a force plate consists of a sheet of wood or metal
sitting atop a grid of crisscrossing beams. Inside each of the beams are gauges that are squeezed by
the pressure of the animal's feet as it walks overhead; some gauges measure the up-and-down impacts
while others sense forward-and- backward or side-to-side forces. But no one had ever tried to use a
force plate to detect the steps of a half-ounce crab before. To do so, Full and a fellow postdoc
gave the plate more sensitive gauges and a more complicated electronic configuration, then sent
crabs scuttling over its surface.
When Full saw the force patterns the crabs were creating, he had a feeling of déjà vu. Far from
moving like a wheel, the crab's center of mass rose and fell, and the crab itself slowed down and
sped up rhythmically. At low speeds it walked like a pendulum, with gravity aiding its forward
movement; and at faster speeds it ran like a pogo stick, with energy clearly being stored and
released in some unidentified springlike structures. The crabs rarely became airborne, which meant
that they, too, could run like Groucho. "I said, 'My goodness, there are so many similarities to the
general model--it can't happen!' " says Full.
To see if perhaps crabs were the exception rather than the rule among arthropods, Full decided to
study cockroaches. But the insects, at a twentieth of an ounce, were too light for even the
state-of-the-art Harvard force plates. Full had to wait until he came to Berkeley later that year,
where he set up the PolyPEDAL lab. It took almost 12 months, but with the help of an undergraduate
named Michael Tu he managed to build and calibrate a force plate that could measure the impact of a
cockroach's steps. The construction wasn't easy. "You have 24 strain gauges made out of silicon
slivers and gold wires about twice the thickness of a hair, and you have to glue all the gauges on
the beams and solder the wires to terminals by hand," Full explains. This was all then covered with
a layer of wood a half-millimeter thick. It was worth the effort, however: Full and Tu promptly
discovered that cockroaches walk and run like crabs--and horses and humans--by behaving like
pendulums and pogo sticks.
But though an arthropod moves like a tetrapod, each of its many legs behaves uniquely. With student
Lena Ting, Full measured the force of each individual cockroach leg by covering the lab's force
plate with a second thin sheet of wood, suspended just slightly above the first, with two cutout
patches the size of a roach's foot. Into these holes went cardboard plugs that sat on the lower
plate. The force of the roach's footsteps was thus detected only when the insect hit the plugs and
indirectly struck the plate below. Using this device, Full and Ting found that when a running
roach's rear legs strike the ground, they push back against it to propel the roach forward; the
front legs act like a set of brakes, decelerating the roach; and the middle legs act like human
legs-- like pogo sticks--to both accelerate and decelerate the body.
It was during these experiments that Full and his students discovered that the American cockroach
can run five feet per second, a speed the Guinness Book of World Records recognized as the fastest
of any insect on Earth. This means the roach travels 50 body lengths in a second. A human would have
to run 200 miles an hour to match that pace.
It was also during these experiments that the researchers noticed something truly peculiar: the
roach was running with its body tilted up 23 degrees. "The film wasn't very good, and we couldn't
tell what the hell the animal was doing," Full remembers. "And then we looked at the force
platform." Periodically it registered no forces at all--in other words, there were times when the
roach had taken all its feet off the ground. Taken together with the images of the roaches tilting
upward, this suggested that the bugs were getting up on their two long rear legs and running. "We
thought the plate was broken, or maybe there had been a gust of wind, so we made the roaches do it
again." Eventually they looked at 40 roaches, using high-speed video with better resolution. In
every case, they could see the roaches tilt up their bodies and sprint on their two back legs, just
like humans.
Roaches turn bipedal at top speeds, Full suspects, because running on six becomes counterproductive.
Their legs, he notes, are moving back and forth 27 times a second, which is probably as fast as
their muscles can work. "The only way to go faster, then, is to take bigger steps," says Full, "and
since the front legs are a third shorter than the rear ones, they can't possibly have the same
stride length."
Still, it was hard to accept that cockroaches could run on two legs, particularly because it
seemed as if they were breaking the laws of physics. "When we calculated it, it seemed that they
should just fall forward because they're tipped so far over," says Full. Their secret lies in
their aerodynamics, a peculiar consequence of the insect's small size and high running speed.
Berkeley biomechanics expert Mimi Koehl teamed up with Full to do wind-tunnel experiments and
showed that when the roach is sprinting, the air pushes against it so hard that it keeps the roach
from falling down.
It's difficult enough to decipher the mysteries of six-legged cockroaches and eight-legged crabs.
What about a 44-legged centipede? Researchers have long assumed that a crawling centipede, at least,
moves like a wheel--after all, it certainly looks as if it's gliding as smoothly as a chair on
casters. They also thought a centipede's movement was rather uneconomical. Many centipedes bend like
snakes when they move quickly, apparently wasting energy in undulations that could be used to move
the animal forward. At fault, supposedly, was the centipede body plan. Centipede legs extend
diagonally down from a long, flexible body. As each leg twists at the joint to propel the animal
forward, the force also inadvertently swings the body around the joint.
To test these assumptions, PolyPEDAL lab member Bruce Anderson turned to the common Arizona
centipede, a species with a particularly pronounced undulating walk. It's a chilling creature, six
inches long, with an orange body, 44 yellow legs, giant pincer jaws, and a painful bite-- indeed,
Anderson will handle it only with a gigantic pair of tweezers.
When he videotaped the centipede in motion, Anderson found what PolyPEDALists have almost come to
expect: rather than moving smoothly forward like a wheel, the centipede's body accelerates and
decelerates as it travels. He also found that it's a much more economical creature than was once
thought. Anderson implanted electrodes in the centipede's trunk muscles and found that whenever a
segment of its body was curved out to the left, contracting muscles would then begin forcing that
segment to curve to the right. When the segment was curved out to the right, the opposite action
would occur. In other words, the centipede was not passively bending as a result of its anatomy; it
was actively trying to undulate. And the bending seemed to be saving it energy--oxygen-consumption
experiments showed that the centipede actually needed less fuel to move than most other animals of
its weight.
To see how it was saving energy, though, meant seeing what each of the creature's 44 legs was doing,
and force plates just weren't up to the task. For that the lab needed another device, known to the
PolyPEDALists as the Jell-O track. The creation of engineering student Angela Yamauchi, it's a clear
plastic chute with a long, shallow tray of transparent gelatin sitting between two filters, one
below the gelatin, the other just above it. Polarized light shines up from the bottom of the track.
Polarized light consists of photons that have been filtered so they all vibrate in the same plane.
If the light encounters a second filter oriented in the same direction, it can pass right through.
But if you rotate the second filter 90 degrees, as is the case with the filter above the gelatin,
all the light is blocked.
Viewed through the filter above the track, therefore, the clear gelatin looks black. But when an
object--say, the foot of an insect--rests on it, the gelatin's structure is transformed. It begins
to interfere with the light passing through, changing its polarization. Photons that had been
blocked by the second filter are then able to pass through. When an insect ambles down the track,
little splotches of white light thus appear around its feet. By looking at the size of a splotch,
Yamauchi can calculate the size of the force, and by analyzing its shape, she can tell you in what
direction the force is moving.
When Anderson set his centipede on the Jell-O track, he discovered something remarkable. At any
moment, most of the centipede's legs are not on the gelatin--in fact, on average, only four touch
the ground at a time. These four are the legs at the centermost point of the concave side of the
four curves in the centipede's body, two on the left side and two on the right. As the centipede
undulates forward and the next foot in line reaches the center of a bend, the centipede lifts up the
planted foot and sets the next foot down on the same spot. As different legs reach the center of the
curves, they make the entire animal speed up or slow down, with the middle legs supplying most of
the propulsive force.
How does this help the centipede save energy? "I made stick figures of these things until I was
bleary-eyed," says Anderson, but eventually he came up with a theory. Since the centipede is moving
only four legs at a time, rather than 44, it takes less energy overall to operate its legs. This
could pose a problem, of course; fewer legs on the ground means less support for the centipede's
long, flexible body. But by bending the body in the first place--which requires flexing its body
muscles--the centipede keeps its body tight and stops it from sagging.
Of the many arthropods that have made their way into the PolyPEDAL lab, only one actually moves
anything like a wheel. In 1979 Berkeley biologist Roy Caldwell encountered a species of
stomatopod--a small, shrimplike creature--that had washed up on a beach in Panama. Lying on its
back, it pulled its tail to its head to form a hoop and did a backward somersault, letting its back
flop onto the ground. Then it pulled its tail up again and rolled on, slowly making its way back
into the ocean.
When Full and his students videotaped some stomatopods in the lab in 1992, they found that for 40
percent of each "stride," when the creature forms a hoop, it indeed moves exactly like a wheel. But
during the other 60 percent--when it's raising its body up and flopping it down--its center of mass
rises and falls, slows down and speeds up. "It's the ultimate example of something that is close to
a wheel," says Full, "but even there, it has forces that look like a leg--except that now the leg is
the body."
It's not only in the animal world that legs seem to make more sense than wheels. Engineers have long
wanted to make robots with legs for traveling over natural terrain, and robots with six legs or more
make the most sense. There is already a long shopping list of uses for a walking robot, including
exploring the moon and Mars and inspecting toxic waste sites. But today's crop of robots is a long
way from any moon walk. One of the most famous walking robots, a six-legged machine named Attila,
from the laboratory of MIT engineer Rodney Brooks, is a case in point: it's slow and it uses too
much energy. "Attila can go three feet in 30 seconds," explains Mike Binnard, a member of Brooks's
lab. "If you watch a video, it looks okay in fast-forward, but at regular speed it's pretty boring."
The problem, according to Full, is that engineers rely on the old conception of insects as having
simple, wheellike locomotion. But Full has been preaching his new biomechanical gospel to robot
designers for three years now, and they're beginning to take his suggestions to heart. Binnard, for
example, is in the process of building an Attila-style robot with cockroach legs. Using Full's
extensive data on the insect, he replaced its six identical limbs with three differently shaped
pairs of legs: small front legs that act like brakes, longer middle legs that push in both
directions, and even longer rear legs that push the robot forward. The savings in energy should help
it travel five times faster than Attila.
Binnard and the other robot designers are not the only ones benefiting from this collaboration. The
questions they ask have led Full and his PolyPEDAL students into entirely new areas of inquiry. For
instance, Binnard recently asked Kram how he might teach his robot to climb over objects. It's
something robots are particularly bad at: they slowly plant one foot at a time, continually check
their balance, and when they finally pull themselves up, they often strip their gears as they go.
Cockroaches, on the other hand, can race over almost anything. Faced with the question, Kram
realized he didn't actually know how cockroaches do it. So he and Yamauchi built a step for the
Jell-O track, ran roaches over it, and discovered that as the roaches approach the step, they tilt
their front ends up with their middle legs and put their front legs on the ledge. But they don't use
their front legs to pull themselves up; instead they boost themselves over with a strong push of
their back legs.
While Full loves the idea of a scurrying roach robot, his ultimate dream is a mechanical crab. Not
only can a crab race across dry land, it can plunge into crashing surf and continue running
underwater. A robot with a crab's skills could do a number of remarkable things. Consider Marines
trying to land on a beach laced with mines. Crab robots could jump out of the landing craft, run
onshore, hunt the mines down, and set them off. Rockwell International recently gave a contract to
Full and IS Robotics, a Massachusetts robotics company, to design such a robot for the Navy. A robot
crab could also distinguish itself in civilian duty. Researchers surveying the ocean floor or
engineers inspecting underwater construction use propeller-driven, tethered machines. In strong
currents or crashing waves these devices get tossed around and are often rendered useless. A robot
with a crab's stability, on the other hand, could easily work in these inhospitable places.
To build such a robot, however, the researchers need to understand better just how crabs do what
they do. Marlene Martinez, another Berkeley grad student, is therefore collecting data on real
crabs. She's seen crabs that live on rocky beaches wrap their legs around small rocks to resist the
crashing waves, and she's heard of species that live on sandy beaches vibrating their legs to
liquefy the sand so they can jam their limbs down into it and hold on tight. She's observed how,
when a crab walks underwater, it adjusts the angle at which its body points so the water will lift
and support it. This added buoyancy means the crab doesn't need to keep as many legs on the ground
for stability. A crab robot would do well to copy any or all of these strategies.
But no matter how deeply involved the PolyPEDALists get in robot design, their main focus is still
on exploring how flesh-and-blood animals- -or chitin-and-hemolymph arthropods, to be more
precise--move. Despite having discovered the first airborne running ant, for example, Kram is
convinced that ants don't have to leave the ground to run. He thinks they, like crabs and humans,
may be able to imitate Groucho Marx. To prove his hypothesis, he's going to have to design the most
sensitive force plate yet; an ant weighs a thousandth of an ounce. But when he's done, chances are
this research, like much of the other work that goes on in the PolyPEDAL lab, will uncover even more
hidden similarities between ants and ourselves.
"I think what our research has said is that evolution is constrained," says Full. "There are not an
infinite number of ways to move, even though they can look very different. To say that a model of a
pogo stick could apply to this kind of diversity--I can tell you, I never would have believed it
unless I'd collected the data myself."
‹ September 1994
DISCOVER Vol. 22 No. 7 (July 2001) discover.comfeatphysics.htmlOpen ↗
The Physics of Walking: Falling Forward Why humans move like an imperfect pendulum By Robert Kunzig
In what one can only assume is Giovanni Cavagna's funniest home video, Cavagna, a jolly physiologist
from the University of Milan, is standing in an aviator suit in the passenger compartment of an
Airbus A-300. The plane, operated by the European Space Agency, has been cleared of its seats and
filled with scientific gear. Cavagna is grinning and holding a pendulum, which is swinging at a
steady pace. Next to him, his friend and longtime collaborator Norman Heglund is pacing steadily
back and forth on a 10-foot-long platform. The plane is cruising at 30,000 feet or so over the Bay
of Biscay, off Bordeaux, France. NASA has a similar plane called the Vomit Comet.
Abruptly, the Airbus starts to climb- so steeply that the horizon outside goes almost vertical.
Normally at this point the pilot would jam the stick forward and throttle the engines way back,
sending the plane over the top of its parabola and into a screaming dive. For 20 seconds or so, we
would see Cavagna et al. floating around the padded compartment in zero gravity. This time, however,
the pilot throttles back gravity to only 40 percent of its terrestrial value- to around what it is
on Mars. Cavagna stays on his feet, but his pendulum starts swinging in long, slow, sloppy arcs. On
the platform Heglund is now taking long, slow, floating steps. "You feel beautiful at .4 g," Cavagna
says. "Walking on Mars would be great."
Walking on Earth, Cavagna says, is a bit of a struggle- and so is trying to understand the physics
of it. Cavagna's Airbus experiments are but the latest in a long series; he has been studying our
awkward form of locomotion for nearly 40 years. Very early on he figured out our basic strategy: To
save energy, we walk like a pendulum. The problem is we do it badly.
A pendulum is a device that transforms kinetic energy of motion into gravitational potential energy
and back. As it moves through the bottom of its arc, the pendulum's velocity and thus its kinetic
energy- mass times velocity squared divided by two, or mv 2/2- reach a maximum. At the top of its
arc, the pendulum slows to a stop, but at that point the potential energy- mass times gravity times
height- is at its peak. As the pendulum falls back down, potential energy is converted back to
kinetic energy. In a good pendulum the conversion is close to 100 percent, with only a bit of energy
lost to the friction of moving through the air and that of the bearing from which it is hung. One
nudge, and a pendulum keeps swinging a long time.
With each step you walk, you yourself become an inverted pendulum: You pivot around the foot that's
on the ground, as if you were using that leg to pole-vault, and your center of mass, somewhere in
the belly, describes an arc. As you plant a foot on the ground in front of you, the ground exerts a
force back up your leg that slows you down, and you continue slowing as you rise up on that foot to
the top of your arc. At that point your kinetic energy is at a minimum- but your potential energy is
at a maximum. As you fall forward into the next step, that stored potential energy is converted back
into kinetic energy, and you accelerate again.
"If the body were a perfect pendulum- if it could convert the kinetic energy into potential energy
and back without wasting a calorie- walking would be nearly effortless," says Heglund, a
physiologist at the University of Louvain in Belgium. "But you're only 65 percent of a perfect
pendulum." In other words, 35 percent of the energy for each step has to be supplied afresh from the
food you burn. Fish and birds do better: They burn less energy per unit distance than we do, even
though birds are fighting gravity all the time, and fish have to fight their way through a dense
liquid. "So why are we sweating? Where's the work?" asks Cavagna. "It's work we're doing against
ourselves. It's a lack of coordination."
Somewhere in our legs, muscles are pulling against one another, wasting energy as heat. Even after
four decades Cavagna is not sure where the waste happens- but he does know at what point in the
stride. The tip-off came from some experiments that he, Heglund, and Heglund's Louvain colleague
Patrick Willems did with women from Kenya.
Women of the Kikuyu and Luo tribes have a remarkable ability: They can carry on their head a basket
of produce that weighs as much as 70 percent of their body. Heglund tried to match the feat, wearing
a bicycle helmet filled with lead shot; he only got up to 15 percent of his body weight. "When that
much weight gets out of balance, it feels like it's going to rip your head off," he explains.
The African women's most surprising prowess, though, is that they can carry as much as 20 percent of
their weight with no extra effort- that is, without using more oxygen and burning more calories than
when they carry nothing. Puzzled, the researchers had the women walk on a platform that records the
forces exerted by the feet, and thus the kinetic and potential energy at each point of the stride.
There is one point, Cavagna's team found, at which load-bearing Kenyan women do far better than the
rest of us. As we move through the top of one stride and start to fall into the next one, most of us
pause imperceptibly for a few milliseconds: We're falling and losing potential energy, but we're not
yet converting it to increased speed, because muscles in our leg are contracting and fighting the
fall. The Kenyan women do the same thing when they're not carrying a load. But put a heavy weight on
their head, and somehow they are able to shorten or even eliminate this pause- and thus to convert
more of their potential energy into forward motion rather than muscle heat. With no visible change
in their gait, their conversion rate rises from 65 percent to as much as 80 percent. In other words,
they become better pendulums. Unfortunately, they have no idea how they do it.
For most people, the optimum walking speed- the speed at which our kinetic energy is in balance with
our potential energy- is around 3 miles per hour. But short legs slow a walker down, and so does low
gravity. On Mars, at .4g, you would glide along, lifting your legs more easily than you do on Earth
and thus exerting less at any given speed. But you wouldn't be able to walk as fast because you
would be falling much more slowly into each new step. On the moon, at around .17 g, in order for
your kinetic energy to balance your minuscule potential energy, you would have to walk so slowly
that you would hardly move forward at all. In 1969, when Neil Armstrong and Buzz Aldrin took their
giant leaps for mankind, Cavagna wasn't at all surprised to see them bouncing (a kind of running)
rather than walking. He had predicted as much in 1964.
The Airbus results teach one potentially useful lesson, Cavagna says: For a manned mission to Mars,
spacecraft designers might consider pegging their artificial gravity not at 1 g but at the agreeable
.4 g of their destination. Certainly they shouldn't choose 1.5 g's, which the Airbus pilot
re-created for Cavagna's group by flying steeply banked circles. You walk faster in 1.5 g's, but you
feel, well, surprisingly heavy. "You pick up your foot and start to fall forward, and you think
you're going to fall on your nose," Heglund says. The video shows Cavagna jerking along like Charlie
Chaplin and looking none too stable.
The next time Cavagna rides the Airbus, he plans to take 1.5 g's at a run; it will be like running
with a backpack loaded with half his ample body weight. At age 67 and with a bad back, he is defying
doctors to forbid him. "I'm not doing this because it's useful," Cavagna says. "I'm doing it because
it's amusing."
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RELATED WEB SITES:
For a discussion of earlier research on the walking of Kenyan women, see Biomechanics Watch by Carl
Zimmer, in Discover's August 1995 issue; this article is available at www.discover.com.
