Invention of the day: "The backpack that's easier to carry". Using a specially designed bungee suspension system,

...a load weighing 27 kilograms "feels" more than 5 kilograms lighter: the walker uses only as much energy as they would for a normal rigid pack weighing 21.7 kilograms.

The reason, Rome and colleagues report in Nature, is that the bungee pack bounces up and down on the frame exactly out of step with the vertical movements of the walker's body. So these movements cancel out and less energy is wasted shifting the load up and down.

Here's a related article.