As with Nasiri’s device, Simpson and his colleagues’ belt is essentially a spring that connects the legs. “It’s really like tying your shoelaces together but making them elastic,” says Selinger. This may sound like a recipe for disaster, but the volunteers who tried the ‘exotic donut’, as it is called, quickly got used to it. “It makes your legs feel light and fast,” says Hawkes, who worked with Simpson and Selinger on the project. “It’s a lot of fun.”
Thanks to the exotic donation, it was possible to reduce energy costs by around 6% while running. The increase in efficiency surprised the researchers, since it is estimated that swinging the legs accounts for only a small fraction of the energy consumption compared to the high energy costs for lifting off the ground and landing. However, the team showed that Exotendon users could save energy by customizing their walking motion.
The main attitude of runners was to increase their cadence by about 8%. A faster leg swing usually costs more energy, since the leg pendulum system is driven at a frequency that is above its natural swing frequency of around 100 steps per minute. But the Exotendon increases its natural frequency, so that swinging the legs faster uses less energy. In addition, a faster stride is achieved by reducing the height of each mini jump while running, which reduces energy consumption.
Exotendon users were able to automatically adjust their steps based on these complex energy tradeoffs. It’s like the runner’s body saying, “Okay, swinging my legs is cheaper for me now, so I’m actually going to swing my legs faster,” explains Selinger.
To better understand the role of customization, many biomechanics researchers are using human-in-the-loop optimization, in which human subjects wear an adjustable device – one that researchers use to adjust the forces and torques applied and the subject’s response can monitor. “In real time, you can adjust and adjust the parameters of the device that the person is wearing to reduce costs,” says Selinger.
In a report last year, Collins’ team used this approach to study how an ankle machine can improve running efficiency. Runners carried the exoskeleton on a treadmill while computer programs varied the force inputs. They discovered a series of force inputs that resulted in a 15% energy saving. The problem is that the device wasn’t passive – it required plugging into a power supply. Several groups are developing active exoskeletons powered by a portable battery  , but the extra weight lowers the energy savings, says Braun.
While Collins and colleagues considered a spring-operated ankle device – similar to the one they used for walking – the energy savings for runners were only 2%. “We’re not sure why the percentage change was less than when we walked,” says Collins. “There are many reasons to expect that a passive device could be more effective when running because of the springy behavior of the leg. But people are complicated. “