The boots mirror the action of the walker's calf muscle and Achilles tendon, saving energy and showing that there is room for improvement in our already very well-tuned gait.
Previous research had produced similar gains but only by using powered, pneumatic "muscles".
The new device is reported in Nature.
Senior author of the study, Dr Gregory Sawicki, from the joint biomedical engineering department of the University of North Carolina and NC State University, said the unpowered exoskeleton acted "like a catapult".
"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," Dr Sawicki said.
Key to the boots' success is a mechanical clutch, which puts tension on the spring when the foot is touching the ground but leaves it slack when the foot lifts and swings forward through the air.
This clutch is made from a ratchet that engages with each footfall and takes up the slack on the spring; it then locks while the foot is on the ground - allowing the spring to off-load some of the strain on the walker's muscles and tendons - and releases again at the back of the stride.
"The clutch is essential to engage the spring only while the foot is on the ground, allowing it to store and then release elastic energy," explained Dr Sawicki.
Nine participants tested the gadgets, walking on a treadmill under very close monitoring, both with and without the exoskeletons.
The energy saving was small but important, Dr Sawicki said: "A 7% reduction in energy cost is like taking off a 10-pound (4.5kg) backpack, which is significant.
"Though it's surprising that we were able to achieve this advantage over a system strongly shaped by evolution, this study shows that there's still a lot to learn about human biomechanics and a seemingly simple behaviour like walking."
Co-author Dr Steven Collins, from Carnegie Mellon University, said that with some more development, the invention had the potential to help people who have difficulty walking.
"Someday soon we may have simple, lightweight and relatively inexpensive exoskeletons to help us get around, especially if we've been slowed down by injury or aging," Dr Collins said.
Engineers have created unpowered exoskeleton "boots" that use a spring and a ratchet to make human walking 7% more efficient.