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New research from the University of Utah illuminates why humans step on their heels when they walk, unlike most other mammals. Heel-to-toe walking is vastly more efficient, requiring less energy than stepping on the toes or balls of the feet, according to findings published on-line Thursday.

Researchers studied 27 athletic volunteers, measuring the exertion associated with different walking postures. They found the subjects expended 53 percent more energy walking on the balls of their feet than walking on their heels, said senior author David Carrier, a U. professor of biology. Toe walking expended 83 percent more energy.

"Most mammals -- dogs, cats, raccoons -- walk and run around on the balls of their feet. Ungulates like horses and deer run and walk on their tiptoes," Carrier said. But humans are the most efficient walkers on Earth, which may explain our species' evolutionary success. What our hunter-gatherer ancestors lacked in strength and speed, they made up with the ability to cover vast distances.

Bears and great apes are among the few other animals that walk heel-to-toe. Because our closest primate relatives heel-walk without being prolific walkers, Carrier believes that we evolved this walking posture before tree-dwelling ancestral humans left the forests of East Africa 2 millions years ago to explore the savannah.

Why would we evolve an efficient mode of walking before our species began roaming? Carrier speculates heel planting confers key advantages other than efficient walking.

"It is rare among mammals and the other animals that have it are relatively fierce," Carrier said. "It is good for turning and applying lateral force and it may be advantageous in fighting."

His new research, which appears in the March edition of The Journal of Experimental Biology , was conducted with Christopher Cunningham, a U. doctoral student in biology, and zoologist Nadja Schilling and physician Christoph Anders of the Friedrich Schiller University of Jena, Germany. Funding came from the National Science Foundation, Jena, and a German insurance group interested in back pain.

Carrier is a one-time protege of Dennis Bramble, the U. emeritus biologist who made headlines with theories that humans evolved into nature's greatest endurance runners. While four-legged animals are more efficient runners, people can outrun any other animal over the long haul because human physiology is so good at dissipating body heat, Bramble has reported.

The new research drew plaudits from a Bramble collaborator, Harvard's Dan Lieberman, whose own research in mammals' overland locomotion suggests running is embedded in human genes and our ancestors ran on their toes -- a footfall completely different than how we walk. Lieberman recently published findings that humans developed a toe-first running posture to soften the impact on bare feet. Only with the advent of modern footwear have runners landed flat-footed.

"There is compelling evidence that humans and fossil hominins have always walked with a heel strike, and Carrier's work helps us understand this on a number of levels," Lieberman, a professor of human evolutionary biology, wrote in an e-mail. "At some point we also evolved the ability to run with our very odd feet, but using them in a markedly different way without heel striking on a regular basis. So Carrier's analyses nicely complement our recent findings about barefoot running."

So why is a heel-strike better for walking?

Muscle activity in the ankle, knee, hip and back increase when you walk on the balls of your feet or your toes, Carrier said. Strides are shorter and more frequent, but that's not what makes this kind of walking less economical.

Heel-first stepping reduces the body's up-and-down motions, whereas stepping ball-first slows the body and requires more re-acceleration between steps. Heel-first walking also enhances the transfer of movement, turning kinetic energy into stored energy, and back again, in ways that toe walking does not.