Millions of years of evolution have enabled human beings to develop a smooth, energy-efficient way of walking.
But, biomechanical engineers suggested on Wednesday, there's always room for improvement.
They unveiled an unpowered exoskeleton -- a small, light, spring-activated device that fits on the lower leg and reduces the energy cost of walking by around seven percent.
"The difference might seem small, but it makes a significant difference for humans," said Steve Collins of Carnegie Mellon University in Pittsburgh, Pennsylvania.
It's the rough equivalent of an infantryman being able to march seven percent more in terms of time and distance for the same energy expenditure, or a trekker freed of the energy cost of toting a four-kilo (10-pound) backpack, he said.
Exoskeletons -- externally-worn devices intended to augment physical actions or strength -- have a long history.
Back in the late 19th century, pioneering inventors toyed with coiled springs tucked into the heels of cowboy boots, or harnesses with rubber bands that were attached to the legs and sought to make walking easier.
But the weight and design of the gadgets were often an energy drain in themselves.
Partly as a result of such setbacks, exoskeleton design today favours powered devices, but in turn has to struggle with challenges such as battery range and bulk.
The new gadget, described in the science journal Nature, is a carbon-fibre mechanical device weighing about 500 grammes (1.1 pounds) for each leg.
It comprises a spring that is attached at one end to a strap around the top of the calf, and, at the other end, to the heel of the shoe.
When the heel meets the ground, the spring becomes extended, and a tiny mechanical clutch engages to ensure that the spring briefly retains this energy.
When the heel is lifted, the clutch disengages and the spring is released, thus giving up its stored energy and providing discreet help for the calf muscles as they lift the lower leg.
- It's spring time -
There's no motor, no battery and no computer, the inventors said proudly.
"The unpowered exoskeleton is like a catapult," said Collins' team-mate, Gregory Sawicki of North Carolina State University.
"It has a spring that mimics the action of your Achilles' tendon, and works in parallel with your calf muscles to reduce the load placed upon them."
The prototype was developed through tests on nine able-bodied volunteers, who wore an exoskeleton on both calves.
They were placed on a treadmill and filmed in order to find a spring whose tension offered the best savings in energy cost.
The reduction averages 7.2 percent, plus or or minus 2.6 percent, for healthy adults who used it under natural conditions wearing normal athletic shoes. The performance is comparable to savings with powered exoskeletons, according to the paper.
The device has been patented, and the hope is that it can be adapted for people with reduced mobility as a result of injury, stroke or ageing.
Further research is needed to ensure that long-term use of the device does not have an impact on body mechanics elsewhere, Collins said in an email exchange.
"The body is very complex, and it is difficult to predict such effects," he said.
Walking takes up a big chunk of our daily energy use.
"People expend more energy during walking than any other activity of daily life," said the study.
"We take about 10,000 steps per day, or hundreds of millions of steps in a lifetime."
Source: AFP