A “bungee backpack” that uses rubber bands to lighten the load enables people to carry 25 per cent more weight while expending the same amount of energy.

The ergonomic design, developed by an American scientist, reduces the force of the backpack’s load by 86 per cent.

A person carrying 60lb (27kg) in a bungee backpack would use the same amount of energy as someone carrying a load of 48lb (22kg) in an ordinary one, according to a study published today in the journal Nature.

“It’s like carrying an extra 12lb for free,” said Larry Rome, a biology lecturer at Pennsylvania University, who designed the device. He said that an immediate application would be in backpacks carried by schoolchildren — “a well- known cause of musculo- skeletal injury”. Soldiers and emergency service workers would also benefit, because the design made running while carrying a heavy load much more practical.

“Being able to move at relatively high speeds is crucial for many professions, such as firemen, first responders, disaster relief workers and police,” Dr Rome said. “If you have ever tried to run with a heavy backpack, it is almost impossible because of the large shocks to your knees and ankles.”

Dr Rome said last night that the bungee backpack was at the prototype stage but he hoped to have it in production in around two years. It was likely to cost about 25 per cent more than a traditional backpack.

In a conventional backpack, the load is strapped directly to the body and moves up and down with the walking motion. The bungee cords and rails on the new design allow it to remain at a virtually constant height.

Dr Rome said: “The suspended backpack reduces the accelerative forces during the more energetically expensive phase of walking, which is when both legs are simultaneously in contact with the ground and performing mechanical work against each other.”

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