A new film, Augmented, documents Herr’s journey – he received two prosthetic limbs when his feet were amputated after he got lost during a climbing expedition – and that of Ewing and Carty.

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It’s a fascinating story which shows how Herr, who originally had no desire to be a medical engineer, devoted his life to developing better prosthetic limbs when he became dissatisfied with the plaster-of-Paris limbs he was given after his own double amputation.

Herr was not motivated wholly by a desire to help himself, although that was part of it. He had vowed to do something positive with his life because a young rescuer had died while searching for him.

He became obsessed with the idea of making more effective limbs for people – his motivation for studying medical technology.

Herr adapted his original prosthetic limbs to make it easier for him to climb rock faces: he would switch between short limbs and very long ones for different types of climb, and also switch his man-made feet around.

This has made him an evangelist for body augmentation. He believes that prosthetic limbs need not only be used to “fix” damaged ones, but could actually be given to healthy people to improve their performance in certain activities.

“There is no ‘normal’ condition for the human body,” Herr says in the film. “If we can feel our legs and they are motorised, we will be cyborgs. In the future it will be considered an augmentation – we will be better with these new limbs.”

Speaking at a recent webinar hosted by health, medicine, and life sciences media company Stat, Carty says the 2013 Boston Marathon terrorist bombings – as a result of which some of the victims underwent amputation – crystallised his collaboration with Herr.

Such teamwork between surgeons and medical engineers is surprisingly rare, Carty notes. “You would think that these kinds of conversations take place all the time, but historically, surgical management of patients like Jim [Ewing] has existed entirely separately from the prosthetists.

“But here, the development of the technology informed the surgical procedure, and conversely, the evolution of the surgical procedure informed how the technology was developed,” Carty said.

Their main aim was to restore the sense of proprioception – a person’s awareness of their body’s movement and location – to the artificial limb wearer. It tells you how your joints are moving, how fast they are moving, and how hard they are pushing.

Without proprioception, it’s difficult to perform actions like walking without thinking about what you are doing.

Proprioception depends on mechano-sensory receptors in joints, muscles and tendons. Amputees cannot experience proprioception in their prosthetic limbs, as the connections that are necessary to enable it are not made. The brain does receive some information, but the messages are confusing.

“This is a disharmonic relationship,” Carty said. “Patients may perceive the position of their limb in space, but it’s discombobulated, it’s not anatomically correct.”

Herr and Carty decided to optimise the limb that needed amputation so proprioception could be restored through advanced engineering.

The aim was “to enable a seamless integration between electrobiology and human physiology”, Herr says. Essentially, the Ewing Amputation preserves normal signalling between the muscles and the brain, so amputees feel like they are controlling their prosthetic limb naturally.

The key part of the technique is restoring the “agonist-antagonist myoneural interface (AMI)”, which is the way that muscles on the back and front of an arm or leg work together to send a reading of physical space to the brain.

“The muscles interact like an internal pulley system in the arm, but they are severed in an amputation,” Carty said. In a Ewing Amputation, the push-pull dynamics of the muscle relationship are restored through robotic engineering, and information is collected through electrodes and decoded by the brain and body.

Ewing said his body adapted quickly to the new prosthetic, and he used it without thinking about it.

“My brain felt that my foot was still there, and used the muscles accordingly,” he said, noting that his body seems to have developed a strong attachment to the prosthetic.

“It’s brain control in the sense that your body is controlling [the prosthetic limb] in the same way you control your intact limbs,” he said.

Ewing said the pain he used to feel in his stump – which was sometimes excruciating – subsided after he was fitted with the new prosthetic. Now he just gets some minor pain from wearing the actual device.

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He has no trouble walking up stairs without thinking about where his foot is – and has even resumed rock climbing. Using his prosthetic limb, he revisited the rock face where he got his injury and climbed it successfully.

Carty says that the procedure has been applied to 34 limbs on 30 patients, and there is now probably enough evidence to prove that it is safe to move out of research and into general use.

“We are hoping this will be the standard way of doing amputations, and will be available to people worldwide,” he said.

Herr, meanwhile, has taken to pushing the idea of cyborgs in venues like TedX talks. He still uses standard prosthetics rather than those he co-designed, but he believes that healthy people could benefit from choosing to have amputations to better fit themselves for specific tasks, something that he describes as “augmentation”.

“Normalcy is not the pinnacle,” he says.

Ewing, however, says it would be ridiculous to have an amputation that was not medically necessary.

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“Being an amputee is inconvenient,” he says. “For instance, getting up to go to the bathroom in the night – you have to crawl across the floor to the bathroom,” he said. “You’d have to be a fool.”