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Insight

Convincing the brain that prosthetic legs are real

23/09/2016
During this summer’s Paralympic games the carbon fibre legs or “blades” used by lower limb amputee runners have perhaps become one of the most iconic symbols of the games. Although for the varying sports such as running, jumping and other activities different lower-limb sports prostheses are used, they share a single collective aim: they are designed to help Paralympians run faster or jump higher.

Fieldfisher Sponsored Olympic triathlon Champion Andy Lewis, uses a Running Blade designed by Ottobock

However, for those who have prostheses for more everyday uses their replacement limbs need to be able adapt to different situations and perform a variety of functions, not just excel in one discipline – just like an actual leg. So what needs to be done so that a prostheses feels 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 “biomimicity”.

A “passive” ankle-foot prosthesis usually uses flexible elastic like a spring mechanism to imitate the behaviour of the Achilles tendon, storing elastic energy and releasing it before ankle push-off. “Active” prostheses use an additional 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 equilibrium between the biological and the artificial limb. Currently, this mainly applies to walking over ground at a steady pace rather than activities such as climbing stairs.

Other ways to make a prosthesis more like a biological leg and improve the user’s comfort are more simple. They also demontrate how important it is to involve the amputee in the design process. Users of the most advanced bionic ankle available right now have said that 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 flat on the ground  whilst sitting with an extended leg, rather than sticking up uncomfortably at a 90-degree angle (as is the case for the majority of prosthetic feet).

Another issue is how the prosthesis is controlled. Active prostheses now include on-board computers to control the motors and emulate human walking. Prostheses are effectively becoming more and more like wearable robots. Moreover, users can even use interfaces that read signals from the brain or muscles so that they can operate the prosthesis like a real leg just by thinking and moving in their normal way. The following stage being trialled is the use of implantable electrodes that send signals to the brain to give the user tactile feedback so they can actually feel the contact on the prosthesis as if it were their own biological limb, closing the loop between human and machine.

These technological and scientific advances connect the amputee more intimately with their prosthetic limb, meaning more focus can be put on how the prosthesis is embodied. In other words, to what extent does the prosthetic limb feel like part of the biological body and does the brain treat it as such?

There is a good understanding of how our body is mapped in our brain. Both the motor cortex, the movement control centre, and the somatosensory cortex where we process a wide range of touch sensations are organised somatotopically. This essentially means each area of the body corresponds to a specific area of the primary motor and sensory cortices. More importantly, this mapping does not disappear after the loss of a limb.

This means there is an opportunity to connect prostheses, through muscles and peripheral nerves, to the parts of the brain that would have controlled and sensed the biological body part. But it may also allow us to measure embodiment, how effectively the brain accepts the prosthesis as part of the body.

Ultimately this line of research, bringing together cognitive neuroscience and biomedical engineering, is not only important for designing better prostheses but it is a unique opportunity for understanding how the brain creates and maintains the image of our bodies, mechanisms that apply equally to amputees and non-amputees.

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