While powered lower limb prosthetics can greatly improve the mobility of amputees, errors in the technology can also cause users to stumble or fall, say researchers at North Carolina State University, Raleigh, and the University of North Carolina at Chapel Hill. They are examining what happens when these technologies fail, with an eye toward developing more reliable powered prostheses.

The devices work by decoding electrical signals in human muscle into signals that control powered prosthetic limbs, such as telling a prosthetic leg that it needs to step forward or step up onto a staircase. A decoding error may cause the leg to step up instead of forward, which could lead to a fall.

Conducted in a lab setting, the researchers had subjects use a customized above-the-knee prosthetic device programmed to make errors, which allowed Huang’s team to monitor each user’s balance and biomechanics. Users were also asked how stable they felt during each trial.

The researchers discovered that some errors were so minor or short in duration that users didn’t even notice them. But errors that lasted longer, or that occurred when a user’s weight was on the prosthetic limb, were much more noticeable. The researchers also determined that critical, or especially noticeable, errors were also characterized by a large “mechanical work change,” meaning the prosthetic limb thought it had to do significantly more or less work than the user intended.