Rich Travs
Tue, Jan-30-07, 17:18
http://www.nature.com/news/2007/070115/full/070115-9.html
How much energy does it take to move around? You might think
it obvious that animals with long legs would use up less
energy covering a given distance than would those with short
legs. But how much leg length determines the energetic costs
of walking or running is hotly debated by scientists.
A mathematical model developed by Herman Pontzer of
Washington University in St Louis, Missouri, sheds light on
the question, and may even provide an answer to why our
ancestors evolved longer legs than their predecessors some
two million years ago.
The main cost of running or walking comes from the force
needed to support the animal's stance. Scientists know that
bodyweight plays a big role in determining this force. But
what about other physical factors?
Pontzer used basic physics principles to predict how running
speed and leg length would affect the forces exerted by an
animal. Longer legs, for example, reduce the amount of
up-and-down movement in a stride, and so also reduce the
force needed to push down with each step. From the total
force predicted for a given animal, he could calculate an
expected energy use.
To see how well this model works, he put some people, goats
and dogs on a treadmill in his lab, and worked out their
energy usage by measuring how much oxygen remained in their
exhaled air after a stint of exercise. The reality matched up
very nicely with his model, he reports in The Journal of
Experimental Biology. ... The model could also offer
anthropologists a new way to study energy use in our ancient
ancestors. "The fossil record shows that two million years
ago, there was a big increase in leg length in early humans,"
says Pontzer. He suggests that a reason for this increase
could have been the energy saved by having longer legs.
How much energy does it take to move around? You might think
it obvious that animals with long legs would use up less
energy covering a given distance than would those with short
legs. But how much leg length determines the energetic costs
of walking or running is hotly debated by scientists.
A mathematical model developed by Herman Pontzer of
Washington University in St Louis, Missouri, sheds light on
the question, and may even provide an answer to why our
ancestors evolved longer legs than their predecessors some
two million years ago.
The main cost of running or walking comes from the force
needed to support the animal's stance. Scientists know that
bodyweight plays a big role in determining this force. But
what about other physical factors?
Pontzer used basic physics principles to predict how running
speed and leg length would affect the forces exerted by an
animal. Longer legs, for example, reduce the amount of
up-and-down movement in a stride, and so also reduce the
force needed to push down with each step. From the total
force predicted for a given animal, he could calculate an
expected energy use.
To see how well this model works, he put some people, goats
and dogs on a treadmill in his lab, and worked out their
energy usage by measuring how much oxygen remained in their
exhaled air after a stint of exercise. The reality matched up
very nicely with his model, he reports in The Journal of
Experimental Biology. ... The model could also offer
anthropologists a new way to study energy use in our ancient
ancestors. "The fossil record shows that two million years
ago, there was a big increase in leg length in early humans,"
says Pontzer. He suggests that a reason for this increase
could have been the energy saved by having longer legs.