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Running speed and body mass



To all Dinolisters,

Though this study was on humans, it could have
implications to the discussion concerning dinosaur
locomotor speeds:
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Built for the race
Body mass index could be a reason why a sprinter's
muscular physique differs from that of a lean
marathoner.
By Jeannine Stein
Los Angeles Times Staff Writer

July 25, 2005

Compare athletes in a sprint event to those running a
marathon and it's obvious that a runner's body doesn't
take one shape ? sprinters tend to have muscular
builds, and distance runners are more wiry. The key to
the differences, according to a new research study,
may lie in a runner's body mass index.

Runners' abilities have long been measured via how
much oxygen they can deliver to the muscles, but that
doesn't tell the whole story of why their physiques
differ so greatly.

"The trick was to figure out what matters from
running," says Peter Weyand, a biologist and assistant
professor in the kinesiology department at Rice
University in Houston. He was the lead author of the
study that appears this month in the Journal of
Experimental Biology.

He and coauthor Adam Davis, a researcher at the Center
for Human Performance at Texas Medical Center in
Houston, focused on the force with which the body hits
the ground. In running, the arms swing back and forth,
and the body bounces up and down. 

"Once you're up to speed, those things can be done
passively," Weyand says. Once energy is loaded into
the tendons, which are elastic, they do the bulk of
the work. That way, "the energy in the body can be
recycled," he adds. Like a Super Ball, once thrown, it
keeps going.

Sprinters, however, have to apply force against the
ground to increase speed, "and those forces are really
large," Weyand says, about two-and-half times
bodyweight, compared with about one-and-a-half times
for jogging. "If you're going to go fast,'' he says,
"you have to hit the ground faster, and you need more
muscles to generate energy."

But bigger doesn't necessarily mean better. Being too
large impedes the ability to go fast, Weyand says,
"because as you get bigger, there is less force
related to body mass." Doing a series of back flips is
easy for a gymnast, but might be much tougher for a
linebacker.

The study discovered that there may be an ideal body
mass index for runners, based on a relationship
between BMI and how much force runners use at racing
speeds. In addition to studying the height and weight
of 45 of the world's fastest male and female runners
in various races during the last 14 years, the
researchers also had 18 athletic men and women run on
a treadmill. While running at a series of constant
speeds, the forces their feet exerted on the ground
were measured.

Between the two body types there's something of a
physiological trade-off, Weyand says. What more
muscular runners gain in strength they lose in
cardiovascular ability, because they have more body
mass to move around. Skinnier runners have more cardio
capability but less strength. Applying that to the
real world, a firefighter probably needs to be
stronger than he is aerobically fit. 
----------------------------------------------------
Below is the research abstract:
----------------------------------------------------
J Exp Biol. 2005 Jul 15;208(Pt 14):2625-31. 
 
Running performance has a structural basis.

Weyand PG, Davis JA.

Locomotion Laboratory, Kinesiology Department, MS-545,
Rice University, 6100 Main Street, Houston, TX 77005,
USA.

The body sizes of highly adapted human and other
mammalian runners vary in accordance with specific
performance needs. Sprint specialists are relatively
massive and muscular while endurance specialists are
conspicuously limited both in body and in muscle mass.
We hypothesized that the greater body masses of faster
specialists are directly related to the greater ground
support forces required to attain faster running
speeds. Using human runners as a test case, we
obtained mean values for body mass, stature and racing
speed for the world's fastest 45 male and female
specialists, respectively, over the past 14 years
(1990-2003) at each of eight standard track racing
distances from 100 to 10,000 m. Mass-specific ground
support force requirements were estimated from racing
speeds using generalized support force-speed
relationships derived from 18 athletic subjects. We
find a single relationship between mass, stature and
event-specific ground support force requirements that
spans the entire continuum of specializations and
applies both to male and to female runners [body mass
(kg)=mass-specific support force x stature(2) (m) x a
constant; N=16 group means, R(2)=0.97; where the ideal
mass constant, D=10 kg m(-2)]. We conclude that
running performance has a common structural basis.

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Guy Leahy