A backpack that uses bungee cords to suspend the load from the frame reduces the strain on users, University of Pennsylvania researchers say.

The effect "is like carrying 12 extra pounds for free," because the suspended load ergonomic backpack makes 60 pounds feel like 48, biology professor Larry Rome said Wednesday.

It was developed for soldiers and rescue workers so they could move quickly with heavy loads, but will also help students, who often carry hefty books and papers.

In a normal backpack, the vertical movement of the hip with every step strains the user. The frame and load move up and down together, so that the force when walking is twice that when standing still, and running imposes three times the pressure.

Rome and his colleagues reduced the pressure by using bungee cords to suspend the sack carrying the load from the frame.

"Essentially, the bungee cords permit the load to stay at nearly constant height from the ground while the wearer walks or runs around it," Rome said.

"What is striking is that you can feel the 86 per cent reduction in force with every step."

As well as easing movement and allowing the wearer to carry a larger load when moving quickly, the backpack reduces the risk of orthopedic and muscular injury, a summary of the research published in the journal Nature said.

Rome, a muscle expert, last year introduced a backpack that generates enough electricity to power several portable electronic devices at once.