Humans adjust walking style for energy efficiency

When it comes to walking, humans know how to take it easy. People can adjust their strolling style on the fly to save energy, researchers report September 10 in Current Biology.

The findings support the long-held idea that people pick the laziest way to move, says Alaa Ahmed, a neuroscientist at the University of Colorado Boulder. “It finally provides us with direct evidence that humans choose to walk in a way that reduces metabolic cost.”

But just walking slowly isn’t enough, says study coauthor Jessica Selinger, a neuromechanist at Simon Fraser University in Burnaby, British Columbia. To be efficient, humans have to find the right balance between a host of different variables that influence gait, including speed, step frequency and step width.

Selinger and colleagues used a robotic exoskeleton that fits on the legs like a knee brace, to tinker with people’s strides. The team let nine volunteers outfitted with exoskeletons walk on a treadmill for 12 minutes to determine their step rates. Then the researchers switched the exoskeletons on.

The device applied resistance to the volunteers’ legs. Depending on the computer program Selinger and colleagues used, it was easier for people to take short, fast steps or long, slow ones. At first, people stuck with their original step rates, even if they had to work harder.

Using a metronome as a prompt, the team then asked volunteers to walk to a variety of beats to vary their step rates. When the team turned the metronome off, volunteers quickly adopted the step rate that was most efficient. 

“They discovered the cheapest way to move,” Selinger says. Some scientists had theorized that this ability might develop over a lifetime, after people had years of practice learning how to move their bodies. But Selinger doesn’t think that theory completely fits.

Human bodies can figure out the most cost-effective way to walk, she says: “It’s something that we’re able to sense and tune in real time.”

The work could inspire new rehabilitation tools for stroke patients who have lost some ability to walk, Ahmed says. For example, instead of telling patients how to move, therapists could strap on an exoskeleton that trains patients’ muscles directly. The body’s tendency to save energy also may help explain why some patients have irregular gaits.