Exoskeletons are a common feature in the natural world. But in recent years, scientists have started experimenting with adding them to humans. Powered exoskeletons hold the prospect of helping people with mobility problems resume a normal life. And there's always the prospect of giving ourselves super-human strength, like Ripley in Aliens. Even without power, an exoskeleton can redistribute the energy from our normal motions more efficiently.

But no two people are quite the same—they differ not only in physical proportions, but also in strides or styles of walking. So how do you match your exoskeleton to a user's peculiarities?

The answer, according to a team at Carnegie Mellon University, is the combination of a genetic algorithm and a treadmill. After a few rounds of optimization, a powered ankle assist had most users walking in an energy-efficient manner. And, by changing the conditions, it learned how to help people walk uphill or carry heavier loads, too.

It’s all in the ankle

As you can see above, the exoskeleton being tested only covers the foot and lower leg. It uses a bit of power to take some of the effort out of walking by adding torque to the ankle during a stride. While there's not a lot to it mechanically, the exoskeleton has four parameters that can be easily tweaked: the timing of when it reverses direction, the amount of torque applied at its peak, and when in the stride that peak is reached. While intuiting how to set these might be possible, there's no guarantee our intuitions will be right or that the same parameters will work for everyone.

So, the team turned to what they're calling "human in the loop optimization." This involves putting an exoskeleton-wearing human onto a treadmill and tracking their metabolic activity to get a sense of how much effort they're expending to walk.

Over the course of an hour on the treadmill, an algorithm runs the exoskeleton, testing various combinations of the four parameters and tracking the metabolic exertion that results. After a round of testing, a genetic algorithm picked the best performers and used aspects of each to create a new generation for testing. The hour was enough in most cases to go through four generations of control software. Taking things further generations led to minimal improvements.

In the 11 individuals this was tested on, the energy used for walking dropped by anywhere from 14 percent to nearly 40 percent, even though they were only wearing one of them. The low figure (14 percent) is about the best that had previously been achieved with unoptimized control software, and that was when the test subject was wearing an exoskeleton on each leg. At the high end, it beat the best performance of hardware that covered the entirety of both legs from the hips down.

Looking at the final control software, there were some things that worked for everyone tested. The peak torque, for example, was always best at mid-stride. But the timing of when the torque was first applied ended up set over a range of about 20 percent of the stride. Intriguingly, in many cases, the optimized control software used less energy than an unoptimized version. So, an effective exoskeleton doesn't necessarily mean pumping a lot more energy into someone's legs.

Context is everything

Since the genetic algorithm produced such effective control software for walking, the team turned it loose on a number of other tasks. The researchers were able to create distinct control software for walking faster, running, walking uphill, and carrying a heavy load. All of these saved energy compared to the same activity done using normal shoes (reductions ranged from 30 percent for fast walking to only four percent for carrying something heavy). It also worked with the hardware on both legs.

The researchers also found that they could optimize for things other than energy expenditures. In a separate test, they minimized calf-muscle activity during walking by over 35 percent.

That may seem like an odd goal, but it's worth stepping back and remembering why we're making exoskeletons in the first place. The goal is primarily to help people who have limited mobility due to injury or disease. The exoskeletons have the potential to both make them more mobile and to actively assist in their physical therapy. It's not hard to imagine a situation where minimizing calf-muscle activity could provide a significant assist to someone's physical therapy.

More generally, injuries and disease are an added layer of complexity on top of the normal variations in gait. This new approach can not only help accommodate that complexity, but it can manage that complexity under conditions ranging from a slow walk to a run.

Science, 2017. DOI: 10.1126/science.aal5054 (About DOIs).