Trying to update stairs is like reinventing the wheel: Improving on a simple design that has withstood the test of time is no easy task. For people with injuries or limited mobility, however, climbing a staircase is no easy task either. By hacking the physics of stair navigation, a team of biomechanical researchers have invented a prototype that could help propel stairs, and their users, to new heights.
“My mom likes to complain that she’s really active and she can walk over long distances, but every time there’s stairs she has difficulty climbing them,” says Karen Liu, a professor of computer science at the Georgia Institute of Technology.
Liu, also the corresponding author on a study in PLOS ONE that describes the new stair prototype, recognized that climbing stairs is ranked as one of the most difficult activities for the elderly or injured. Even when individuals are otherwise physically healthy and mobile, losing the ability to navigate stairs is often the deciding factor that forces people out of their homes and into assisted living communities.
The new stair prototype harnesses energy when a person descends the stairs, and recycles it on the way back up to give users a boost. It works because, counter-intuitively, people waste more energy going down stairs than up.
When climbing the stairs, all the energy you put into hefting your legs gets turned into potential energy—when you get to the top you’re higher than you were before. Descending the stairs is another story. With every step your body is essentially in a controlled fall, and the energy your muscles expend to prevent a nasty tumble to the bottom is wasted. Liu thought that wasted energy could be captured and returned on the upward climb.
“There was no way I could create a device like that,” Liu said. As a computer scientist, she had a good idea but no way to actually build a prototype. She turned to Lena Ting, professor of biomedical engineering at Emory University and an expert in human kinetics. “No one knows movement better than Lena,” she says. Together with Ting’s then post-doctoral researcher Yun Seong Song, they put their heads together and refined a design.
To their surprise, reinventing the wheel was a piece of cake. “It wasn’t as difficult as I thought it would be,” Song says. “We aimed for simplicity—we weren’t going for a fancy robot that can move and talk and make good decisions.” They were just trying to hack into the mechanics that already happen every time you go up or down a flight of stairs.
Each tread of their prototype energy-recycling assistive stairs are attached to springs. On the way down the stairs, pressure from each footstep pushes down a moveable platform, compresses the springs and locks the tread in place with an electromagnetic lock. The compressed springs capture and hold on to the energy that would otherwise be dissipated. On the way back up, a pressure gauge on each tread senses the foot, releases the lock, and the springs release that stored energy to help propel the climb.
“I thought, there’s no way this is going to work,” recalls Ting. Spring-loaded steps brought to mind visions of catapults or slingshots. That’s fine if you’re an Olympic vaulter, but not if you’re an elderly or injured person just trying to make your way upstairs.
They went through several spring designs and settled on a gentle extension spring. If you stand still on their new stairs, you don’t have to worry about being catapulted to the next landing. It’s not until you’re in motion that the springed platforms gently assists your foot up to the next level. “It can only help you as you’re actively going up or down, it can’t push you around,” Ting says.
Walking down their stairs “feels like walking down a hill with very soft soil,” Song says. “It’s like you have a cushion at every step, and as you walk down you’re squishing them. You feel low gravity.” That sensation was easy to master for the test subjects they brought into the lab.
Going up on the other hand, “It’s like someone is actually lifting your foot,” Song says. Everyone has squished through soggy grass in the rain, but feeling a ghostly assistance as you walk up the steps? That takes some getting used to. Still, after over 300 test runs, they reported no issues with safety. They also used sensors to measure joint activity, and showed that ascending the assistive stairs required significantly less work than what’s expended on conventional stairs. What’s more, their prototype comes together for a fraction of the price of installing an elevator or stairlift.
With these promising results, the team is hopeful they’ve got a product that could be marketed to make life easier for the elderly or injured. “Energy recycling stairs are a great idea,” says Steve Collins, professor of mechanical engineering at Carnegie Mellon University who was not involved with the study.
“Our muscles are amazing things,” Collins says. They grow stronger to suit our needs, they run off fuel we supply them from the environment, and they heal themselves. “Any engineer would love to be able to do some of those things,” he says, but for all their amazing capabilities, one thing muscles are bad at is recycling energy. With this new staircase, Collins says, the inventors have hacked the physics to do what our muscles can’t. “They efficiently capture and return the energy to you.”
Because the stairs are easy and inexpensive to install, Collins thinks they could allow older people or those with limited mobility to stay in their homes a bit longer. “It could really make a difference for a lot of people,” he says. “With some little tweaks...I think this could be a good product.”
Liu and her team have a provisional patent on their assistive stairs, but marketing their invention will depend on interest. They’ve got a lot of ideas, and could even implement a suite of new features for their stairs like storing the captured energy for use in other applications. Instead of help ascending the stairs, for example, they could jerry-rig a mechanism that could shuttle that energy to charge a cell phone instead.
Next on their docket is simply developing a full set of stairs that will build upon the existing prototype, however. At the end of the day, their goal is to help those with impaired mobility stay in their homes. Liu recounted showing her mom their prototype stairs. “Her comment was, ‘Well, you had better hurry.’"