"Our heel touches the ground at the start of each step, " said Professor David Carrier of the University of Utah, senior author of the new study.

"In most mammals, the heel remains elevated during walking and running."

"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." he adds.

"Few species land on their heel -- bears and humans and other great apes [such as] chimps, gorillas, orang-utans."

"Our study shows that the heel-down posture increases the economy of walking but not the economy of running."

The major findings of the study include:

• "The activity of the major muscles of the ankle, knee, hip and back all increase if you walk on the balls of your feet or your toes as opposed to landing on your heels," said Professor Carrier. "That tells us the muscles increase the amount of work they are producing if you walk on the balls of your feet."
• "When we walk on the balls of our feet, we take shorter, more frequent strides," said Professor Carrier. "But this did not make walking less economical." Putting the heel down first and pivoting onto the ball of the foot makes the stride longer because the full length of the foot is added to the length of the step. But that has no effect on energy use.
• The researchers wondered if stepping first on the balls of the feet took more energy than walking heel-first because people are less stable on their toes or balls of the feet. But increased stability did not explain why heel-first walking uses less energy.
• Stepping heel-first reduced the up-and-down motion of the body's center of mass during walking and required less work by the hips, knees and ankles. Stepping first onto the balls of the feet slows the body more and requires more re-acceleration.
• Heels-first steps also made walking more economical by increasing the transfer of movement or "kinetic" energy to stored or "potential" energy and back again. As a person starts to step forward and downward, stored energy is changed to motion or kinetic energy. Then, as weight shifts onto the foot and the person moved forward and upward, their speed slows down, so the kinetic energy of motion is converted back into stored or potential energy. The study found that stepping first onto the balls of the feet made this energy exchange less efficient that walking heels-first.
• Heel-first walking also reduced the "ground reaction force moment" at the ankle. That means stepping first onto the ball of the foot "decreases the leverage, decreases the mechanical advantage" compared with walking heel-first, Professor Carrier said.

But if heel-first walking is so economical, why do so many animals walk other ways?

"They are adapted for running," said Professor Carrier.

"They've compromised their economy of walking for the economy of running."

"We are not efficient runners."

"In fact, we consume more energy to run than the typical mammal our size."

"But we are exceptionally economical walkers."

"This study suggests that one of the things that may explain such economy is the unusual structure of our foot."

"The whole foot contacts the ground when we walk."

"We have a big heel."

"Our big toe is as long as our other toes and is much more robust."

"Our big toe also is parallel to and right next to the second toe."

"These features are distinct among apes, and provide the mechanical basis for economical walking."

"No other primate or mammal could fit into human shoes."