To save your energy while strolling, walk this sway

To save your energy while strolling, walk this
2 February 2015, by Pam Frost Gorder
of mechanical engineering and director of the
Movement Lab at Ohio State.
"What we found is that when there are only a few
people on the bridge, it's energetically optimal to
walk without shaking the bridge. And if there are
enough people on the bridge, it is better to shake
the bridge, and thereby lower your energy costs,"
Srinivasan said.
Researchers at The Ohio State University have
discovered that walking on a swaying surface consumes
5 percent less energy than walking on a stationary
surface. The swaying of the original London Millennium
Footbridge in 2000, which led to the bridge's closing and
subsequent redesign, was the inspiration for the study.
Here the redesigned (and non-swaying) bridge is shown
in 2014. Credit: Emily Caldwell, Courtesy of The Ohio
State University.
When participants in a charity event took the first
walk across the newly opened London Millennium
Footbridge in 2000, their feet fell into sync, and the
natural side-to-side motion of their steps caused
the suspension bridge to sway.
That's the physics of what happened that day, but
a study at The Ohio State University is trying to
explain the human side of the phenomenon:
Why—consciously or unconsciously—did people fall
into the same cadence, and keep walking that way,
even as the bridge swayed beneath them?
The swaying unnerved many of the walkers that
day, causing officials to close the bridge for two
years to make modifications.
But the walkers did it to themselves, because
walking on a swaying surface saves energy—about
5 percent—compared to walking on a stationary
surface, said Manoj Srinivasan, assistant professor
The study appears in the February 2015 issue of
the journal Proceedings of the Royal Society A.
Before engineers added dampers to the bridge and
re-opened it in 2002, the structure had a certain
"springiness" to it, not unlike a treadmill, he
explained. But unlike a stationary treadmill, the
bridge was able to swing side to side on its steel
cables. Something about that situation encouraged
people to walk a certain way.
Enter Srinivasan and his team, who are trying to
develop a complete theory of legged locomotion to
explain why people walk the way we do. Stability, of
course, is a primary concern when we walk. But
beyond that, research in the Movement Lab is
showing that people's next top priority appears to
be conserving energy, regardless of the situation.
It's an effect that he and his colleagues have begun
to jokingly call "the principle of maximum laziness."
People tend to adjust factors like cadence and the
length and width of their strides if doing so will save
even a tiny amount of energy.
"Of course people have to be stable, but once they
are stable, they want to move in a manner that
makes them the least tired," said Varun Joshi,
doctoral student and lead author of the article. "Five
percent doesn't sound like a lot of energy savings,
but it's about equivalent to walking with or without a
school backpack. It's not a huge burden, but
something people notice."
Ultimately, Joshi and Srinivasan would like to test
how people move on a swaying surface in a
laboratory setting, though that would be challenging
to set up. But for this analysis, they created a
computer model to do the same thing—a simplified
human body walking on a flat surface. They
compared the model walking on a typical treadmill
surface to one with the springiness and swaying
capability of the original Millennium Bridge design.
More information: Proceedings of the Royal
Society A,
Provided by The Ohio State University
On the typical treadmill, the model human bobbed
up and down as we normally do when we walk.
That is how we expend most of our energy when
we walk—pushing off from the ground. But on the
treadmill that swayed side to side, the model
bobbed a little less to achieve the same forward
motion, so that less effort was needed by the legs,
and about 5 percent of energy saved.
The frequency of the swaying and the properties of
the bridge have to be just right for the energy
reduction to happen.
Part of why Srinivasan is so fascinated with the
Millennium Bridge debut is that people somehow
spontaneously fell into a cadence that created a
such a sway, which caused more people to fall into
the same cadence, which made the bridge sway
more. The people on the bridge that day seemed to
widen their stride, unconsciously increasing the
side-to-side forces that in turn increased the
"Ultimately, the behavior is probably a tradeoff
between stability and energy. Taking wider steps
can increase stability, but it turns out that when
you're walking on a swaying bridge, taking wider
steps also saves energy," he said.
Much more work needs to be done before
researchers can completely understand all the
forces that cause us to walk the way we do, but the
answers would help with the design of prosthetics
and other assistive devices, as well as robots.
APA citation: To save your energy while strolling, walk this sway (2015, February 2) retrieved 6 February
2015 from
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