Imagine being tripped over and over again, knowing that it would keep happening, but never knowing when. Nightmarish, right?

That’s exactly what some people volunteered to do in order to help make prosthetic legs better.

Prosthetic limbs have improved in recent years, with some that use microcontrollers, software, and a viscous fluid that can dampen the force of the foot landing on the ground.

But one thing that all prosthetic legs are still bad at is tripping. People with an above-the-knee amputation are 200 times more likely to fall than their age-matched peers with two legs.

“So, there’s a lot higher risk of falling and then subsequently, a lot higher risk of injuries that come out of falling,” said Vanderbilt University graduate student Shane King.

King and his colleagues designed a system that would help them study the mechanics of tripping and falling. They wanted to observe how able-bodied individuals react to tripping and usually manage to stay upright. In June, they published a study about their work in the Journal of NeuroEngineering and Rehabilitation.

The problem: people are very good at not tripping. An early system dropped an obstacle onto a treadmill, but not in a smooth way. Even though the test subjects couldn't see the obstacle, they could feel a slight vibration.

“People would just step right over it,” King said.

So, the researchers designed an acrylic track that smoothly delivers a 35-pound steel block onto the treadmill, no vibration. They added headphones, blinders, and a safety harness. And one more thing — the test subject was asked to count backwards by sevens starting at a very large number.

“It is always everyone's least favorite part,” King said. “It keeps them really preoccupied, though.”

They first tried the whole system on themselves a few hundred times. One of King’s lab mates was tripped 70 times in one day.

“The anxiety goes away, so you're not worried about it,” he said. “It definitely gets funny going back and watching the videos of ourselves doing it.”

King, who has both his legs, also tried on a device designed to allow an able-bodied person to experience using a prosthetic leg. (Basically, the knee is bent and the prosthetic acts as the lower leg.)

“I was so nervous that the first time when I fell, I think my heart rate reached 180 beats per minute,” said King. “It felt like learning to fall as an adult if you never learned to fall as a kid.”

In the next phase, the researchers will use what they learned to program computer-controlled prosthetic legs. Then they will compare their new devices to the ones that people use now. That will involve asking actual amputees to be tripped on the treadmill.

“We want to get everything working as well as we possibly can before we start to bring participants,” King said. “We want to make sure that we're not wasting their time.”