When you walk around carrying a heavy backpack, the movement of the pack against your body increases the amount of energy you need to take each step. Now, Huazhong University of Science and Technology professor Caihua Xiong and colleagues developed a motorized backpack that automatically adjusts the load you're carrying. From IEEE Spectrum:
“Asian people utilized flexible bamboo poles to carry bulky goods, and Romans designed suspended backpacks to carry heavy loads, which show energetic benefits,” he notes. “These designed passive carrying tools have the same principle [as ours].”[…]
In [the motorized backpack's] passive mode, two elastic ropes symmetrically arranged balance the weight of load within the backpack. Or, the user can choose to turn the system into active mode, whereby a rotary motor regulates the acceleration of load. The whole backpack weighs 5.3 kg, and was designed to carry loads up to 30 kg[…]
Results show that, the motorized backpack in active mode reduces the load acceleration by 98.5% on average. In terms of metabolic costs for the user, motorized backpack design required on average 8% and 11% less energy, in passive and active modes respectively, compared to the standard rucksack.