Prosthetic Devices: The Mystery of Human Design

In the course of an average day I go up and down the stairs in my apartment building, I walk to class, and I run errands – all on foot. Not having a car, or even a bike drives home just how much I rely on my legs to get me where I need to go. But what would I do, if simply getting up in the morning and walking to my destination wasn’t possible?

For millions of people in the world, it isn’t. When people say that we should appreciate our health, I think of viruses, cancer, heart disease, or mental illness – I rarely think of the fact that I have all of the parts of my body that I’m supposed to have. For those individuals who don’t have their legs either by a birth defect, traumatic accident or as a result of war, the loss of mobility changes everything about how you would go through your day.

via Wikimedia Commons

The leg, knee, ankle, and foot (the lower extremity) perform two biological functions – stability and mobility. The lower extremity is designed to hold the body’s weight. According to Dr. Mark Geil, Director of the biomechanics laboratory in the department of kinesiology and health at Georgia State University, the lower extremity is amazing in its ability to support the body given our height and the relatively small surface area provided by the foot. In addition to stability, the muscles in the leg are key to making us mobile at various speeds, and over a variety of terrains and conditions.

For people who don’t have or have lost their leg, replicating the stability and mobility of the natural leg is the challenge presented to the researchers who design prosthetic devices. Prosthetics are artificial legs made of metal and plastic that take the place of the lost limb. But what mechanical challenges does replicating the human body entail?

The process of creating an artificial limb that acts like a human leg is called biomimetic design. According to Dr. Geil, just the act of walking can be incredibly complex, so researchers study it to inform their designs. In some cases, the goal is not to get the artificial leg as anatomically similar to a real leg as possible – it is to get the functionality as similar as possible.

Dr. Geil gives the example of a foot, which uses controlled motion through eccentric action of a muscle, called the tibialis anterior, to absorb shock every time we take a step. A prosthetic foot/ankle unit has no muscles, so the function of shock absorption has to be replicated through materials built into the artificial foot that essentially has a solid, immobile ankle.

But are there elements of human design that cannot be replicated by the ingenuity, creativity, and dedication of researchers? The answer is quite simply, yes – muscles.

“Muscles are the only tissue that can actively produce force,” says Geil. “They are not just springs that can store energy and return it, they actively produce energy. There are a few powered prosthetic components available now, and they generate force, but they must do so via a heavy motor and batteries. Nothing comes close to the elegance and efficiency of muscle action.”

via US Army Flickr

According to Dr. Geil, replicating human anatomy and function is the “Holy Grail” for biomimetic design. But finding a way around the problem posed by muscles is only part of the problem. The human leg is designed to accomplish a vast array of activities – and this diversity has proved difficult to make possible with a single prosthetic leg.

“We think nothing of ascending and descending stairs, walking slowly or quickly and sometimes running, stopping, turning, sitting and standing. We walk on varied surfaces and up and down ramps. We climb ladders or kneel,” says Geil. “I believe that something we’re still missing, and something that might be designed into future prosthetics, is the adaptability required for different conditions and tasks.”

While the technology needed to give people who have lost a limb a prosthetic that can do all the things that a natural leg would be able to do isn’t available yet, there is progress being made to understand the way the human body works. The more researchers know about the body – the better they will be at making machines the act like the human body.

For all of human history we have been studying ourselves, trying to figure out how these bodies work – and there is still so much to know. The next time I take the stairs in my apartment building, and walk the few blocks to campus, I know I’ll have a new appreciation for the legs that are getting me there. Not just because I have them, when so many others don’t, but because I appreciate how beautifully, and still mysteriously, they are designed.