The carbon fiber legs or “blades” used by lower limb amputee runners have arguably become one of the most iconic symbols of the Paralympic Games. Although different lower-limb sports prostheses are used for running, jumping, and other activities, they share a single common aim: they are designed to help paralympians run faster and jump higher or further than other competitors. Form follows function.
For those who have prostheses for more everyday uses, however, their replacement limbs need to be able to adapt to different scenarios and perform a variety of functions, not just excel in one discipline — just like an actual leg. So how can we make prostheses feel more like the real thing rather than a specialist tool?
Whereas modern running blades have a distinctive hook shape, one of the most promising engineering approaches for everyday prostheses is to closely model the biological design of a leg, ankle and foot. This approach is referred to as “biomimicry.”
A “passive” ankle-foot prosthesis generally uses elastic like a spring to replicate the behavior of the Achilles tendon, storing elastic energy and releasing it before ankle push-off. “Active” prostheses additionally use an actuator or motor to make up for the power previously provided by the calf muscle at every step. Such prostheses have been shown to help users walk more like a non-amputee and improve symmetry between the biological and the artificial limb. At the moment, this mainly applies to walking overground at steady speeds rather than activities such as climbing stairs.
Simple design, advanced technology
Other ways to make a prosthesis more like a biological leg and improve the user’s comfort are more simple. They also illustrate how important it is to involve the amputee in the design process. One user of the most advanced bionic ankle currently available told me its greatest feature was not that it provided a powered push-off or that it allowed them to walk more like a non-amputee. Instead, it was that the foot dropped flaton the ground when sitting with an outstretched leg, rather than sticking up awkwardly at a 90-degree angle (as is the case for the majority of prosthetic feet).