Gait Study Measures Energy Costs

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http://www.sciencenews.org/articles/20050917/fob7.asp

For 200,000 or more years, the fine-tuned mechanics of human
motion have enabled our species to traverse enormous distances
on foot with remarkably little energy expenditure. Scientists
have long pondered which specific qualities of walking and
running render those means of getting around so efficient.

Now, a pair of biomechanics theorists say that they've captured
the essence of what makes human locomotion as thrifty as it is.
Their model explains why any number of silly-looking gaits, such
as the crouching strut of Groucho Marx, tire us out more than
routine styles of perambulation do, says Manoj Srinivasan of
Cornell University.

The analysis by Srinivasan and Andy Ruina, also of Cornell, offers
a new "conceptual framework" for human locomotion, says Arthur D.
Kuo of the University of Michigan in Ann Arbor.
...
Srinivasan and Ruina used a computer to create an exceptionally simple
model of bipedal motion in which pistonlike legs propel the upper body,
represented as a pointlike mass, through space along perfect arcs.

The model, to be unveiled in an upcoming issue of Nature, assumes that
a moving bipedal creature uses metabolic energy only when pushing
against the ground at the start and finish of each of stride, Ruina
explains.

In between, a walker's foot becomes a fixed point around which the leg
swings like an upside-down pendulum. Just as a regular pendulum traces
a circular arc with extraordinary energy efficiency, so, too, does this
leg motion, the researchers propose. In running, the model shows that
the energy efficiency derives mostly from the fact that the runner
spends considerable time flying through the air, at no energy cost,
along parabolic arcs.

When Ruina and Srinivasan simulated a wide range of possible gaits based
on these fundamentals, they found that ordinary walking and running were
two of the three most-efficient gaits.

The third efficient stride was a peculiar lope halfway between walking and
running that resembles no gait known to biomechanics specialists. People
with certain disabilities might use it without the gait having been
previously recognized, the Cornell scientists suggest. On the other hand,
the hybrid gait might be an artifact of the model's extreme simplicity,
Srinivasan admits.
...

The Cornell link is here:


http://ruina.tam.cornell.edu/research/topics/locomotion_and_robotics/papers/WhyWalkandRun/index.htm

Pre print pdf here

http://ruina.tam.cornell.edu/research/topics/locomotion_and_robotics/papers/WhyWalkandRun/WhyWalkRun.pdf

>From the "abstract"

Although people?s legs are capable of a broad range of muscle-use and gait
patterns, they generally prefer just two. They walk, swinging their body over a
relatively straight leg with each step, or run, bouncing up off a bent leg between
aerial phases. Walking feels easiest when going slowly, and running feels easiest
when going faster. More unusual gaits seem more tiring. Perhaps this is because
walking and running use the least energy1-7. Addressing this classic1 conjecture
with experiments2,3 requires comparing walking and running to many other
strange and unpractised gaits. Instead, here we seek basic understanding of gait
choice by calculating energy cost using a minimal mechanics-based model. This
mathematical model can describe walking and running as well as an infinite
variety of other gaits. We use computer optimization to find which gaits are indeed
energetically optimal for this model. At low speeds, the optimization discovers the
classic inverted-pendulum walk8-13. At high speeds, it discovers a bouncing
run12,13,
even without springs. And at intermediate speeds, it finds a new pendular-running
gait that includes walking and running as extreme cases.
.