Mark FloroWrit 340

Illumin PaperThe Science Behind the Blade Runner

For the first time ever, a double amputee was able to participate in the Olympic Games because

of a technology known as the Flex-Foot Cheetah. This technology is the latest in prosthetics

made for sport and was designed through the field of biomedical engineering. Using carbon-

fiber blades, the user is able to mimic the same running motions of a person with two biological

legs. Due to its exposure from the Olympics and the man who they call the “Blade Runner”, the

Cheetah has sparked intrigue among many. However, with this intrigue has also come debate

about the fairness or unfairness of these prosthetic racing legs. Many interesting questions have

been raised about whether or not to limit the sport as well as whether or not the Cheetah

provides amputees with advantages over those with biological limbs.

Introduction:

Just a few months ago, the 2012 Summer Olympics took place in London and as usual, the world

was in left in awe. A jaw dropping 38 world records were broken, showing that we humans have

continued to find new ways to get stronger, faster, and simply better [1]. The London Olympics

were also filled with many firsts, one such being that this was the first Olympics ever where all

countries participating had women athletes on their teams. But perhaps the greatest and most

controversial first, as well as an extraordinary example of how we as humans have not stopped

improving, was the participation of double amputee Oscar Pistorius.

Fans of sprinting may have already been familiar with the name, but for most of us, Oscar

Pistorius was an unknown up until July 5th, 2012, when it was announced that he would be the

first double amputee to run in the Olympic Games. The announcement itself left many excited

that a man separated from his legs was competing among the world’s best sprinters, as well as

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Figure 1: Oscar Pistorius sprinting using his Cheetahs.

http://www.telegraph.co.uk/sport/olympics/news/9515233/Paralympics-2012-Oscar-Pistorius-makes-his-entrance.html

skeptical of the fairness or unfairness of the situation. Not surprisingly, Pistorius and his

prosthetic legs became a huge subject of intrigue.

Born in 1986 in a province of South Africa, Pistorius

was diagnosed with fibular hemimelia, which is the

shortening or absence of the fibula, in both legs [2]. This

led to a double amputation of his legs halfway between

his knees and ankles [2]. Using Flex-Foot Cheetahs, the

artificial feet that spawned the nickname “Blade

Runner”, Pistorius became one of the fastest humans

alive (Fig.1). The engineering behind his “blades” and

the possible advantages they give will be the main focus

of this article.

The History Behind the Blades

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Figure 2: The initial design for the Cheetah feet by Van Phillips.

http://web.mit.edu/invent/iow/phillips.html

Figure 3: Today’s J-shape design and the current model of the Flex-Foot Cheetah.

http://www.ossur.com/?PageID=13462

One of the first major breakthroughs in prosthetic legs was produced from a tragic 1976 incident

that left a man named Van Phillips without his left leg beneath the knee [3]. Fitted with

prosthetics that were stiff, awkward, and generally

uncomfortable, Phillips set out to engineer a new leg that

would allow him to be as active as he was before the accident

[3]. Changing majors

to Biomedical

Engineering and

transferring from ASU

to Northwestern University, he began the initial

conceptualizations of a prosthetic limb that would provide

“strength, resilience, and flexibility [3].” By studying the

hind legs of cheetahs, Phillips designed a prototype that

would be modeled after their C-shape (Fig. 2). In the search for the right material for the leg, he

eventually chose carbon fiber because of its light weight, durability, and high energy return [3].

The final design took more of an L-shape, and was the base product for the beginnings of Flex-

Foot, Inc. Phillip’s products became popular amongst Paralympic athletes quickly, and to this

day Phillip’s company has remained the leading seller of artificial racing limbs [3]. Today’s

design has now progressed into a J-shape and is known as the Flex-Foot Cheetah (Fig. 3) [4].

The Science Behind the Blades – How They Work

The Flex-Foot Cheetah blade is a product designed by Ossur that possesses a simple look while

utilizing principles of force and motion, creating a relatively simple but ingenious design. The

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Figure 4: Graph showing the relationship the force being applied versus the foot is touched to the ground.

http://barefootrunning.fas.harvard.edu/4BiomechanicsofFootStrike.html

main goal of the Cheetah is to allow the person using it to be able to use the same principles and

techniques in sprinting applied by a person with two biological legs [4].

The Way We Sprint

Before you can truly understand how the

Cheetah works, you must understand how

someone with biological limbs runs, as the

Cheetah was designed to recreate the running

motion. According to Sports Illustrated’s David

Epstein, all Olympics sprinters run basically the

same way [5]. When running at top speed, the

sprinter’s leg acts like a piston pump, touching the

foot down for less than a tenth of a second (Fig. 4)

[5]. As the foot makes this brief contact, it is said to be loading up, which means that it is going

through the cycle of initial contact to fully supporting the body [6]. After the foot has been

loaded up, propulsion starts to occur and the foot begins to come off the ground [6]. The

propulsion serves to create a force that is large enough to lift the sprinter’s body back into the air

for slightly more than a 10th of a second [5]. This small interval is just enough time for the

sprinter to bring his or her other leg forward and strike the ground again. Many factors can

contribute to what makes one sprinter faster than another, but the force applied to the ground by

one’s foot is a leading one [5]. The rate at which a sprinter swings his legs through the air is also

important in differentiating him from his rivals [5]. However, according to Epstein, all able-

bodied sprinters swing their legs at about the same rate: roughly one third of a second between

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Figure 5: A simplistic look at how the Flex-

http://blog.newscom.com/?p=10759

strides [5].

The Design

The J-curve of the artificial foot helps to mimic how someone with two biological legs would run

by recreating the above mentioned cycle of the foot touching down and forward propulsion. The

vertical forces of the person wearing the Cheetah

serve to compress the J curve, which stores energy

while bent [4]. This can be likened to the foot

applying pressure to the ground and “loading up”.

Additionally, the J curve design also functions in

helping to absorb the “high levels of stress that would

otherwise be absorbed by the runner’s ankle, knee, hip, and lower back [4].” Once the cycle of

loading up has been finished, the J-curve returns to normal shape. This releases the stored energy

and creates a very similar propulsion that sends the user forward (Fig. 5) [4].

Other aspects include additional layers of carbon at higher stress points and less carbon where

more flexibility is needed, such as the toe portion [4]. On the Flex-Foot official website, there is

an important note about the lack of heel. This is to ensure that the prosthetic foot’s reaction

“accurately mimics that of an able-bodied runner replicating both the stance and swing phases of

running [4].”

What It’s Made Of

The Flex-Foot Cheetah is made of a carbon-fiber-reinforced

polymer [4]. Carbon-fiber-reinforced polymer, more commonly

known as carbon fiber, is a material that has many applications

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Figure 7: Oscar’s stance as the race begins.

in engineering because of its strength and rigidity [7]. A composite material, carbon-fiber-

reinforced polymer is made of two parts; reinforcement and a matrix (Fig. 6) [7].The

reinforcement is the carbon fiber and the matrix is a polymer resin. The carbon fiber provides

strength while the resin binds the reinforcements together [7]. Even though it is an expensive

material to use, it is crucial to the design of the Cheetah and was chosen so that the leg could be

light weight but also durable, as well as flexible but yet still rigid. The foot’s ability to withstand

the pressure of the forces pushing down on it is key and is made possible by the carbon fiber [4].

So Is It fair?

The unique and effective design of the Flex-Foot Cheetah raises interesting questions about the

fairness or unfairness of a sprinter with Cheetah feet versus a sprinter with biological feet.

Several studies have since occurred in order to answer these questions [5].

One such study, by researchers Matthew Bundle and Peter Weyand, provided results that

suggested that all sprinters, whether Olympic status or not, have the same leg swing time at

maximum speed [5], [8],[9]. However, Pistorius’s swing times were measured on a high speed

tread mill and recorded startling numbers. According to the data, Oscar swings his legs between

strides in 0.284 of a second, a whole 20 percent faster than sprinters with normal legs at the same

top speed [5], [8]. Weyand suggests that this is because his legs are 20 percent lighter than intact-

limbed sprinters [5]. Because Pistorius can make up time with his rapid leg swing, he can leave

his foot in contact with the ground longer than other sprinters, which allows for more time to

apply force to the ground to gain speed. Thus, to attain the

same speed of other runners, Pistorius only need apply about

20 percent lower force to the ground [5], [9]. That would

seem like a clear advantage. But with the advantages also

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Figure 7: Oscar’s stance as the race begins.

come drawbacks. Sports Illustrated’s David Epstein points out one disadvantage of using the

Flex-Foot Cheetah; the fact that Pistorius must stand up straight at the start of a race (Fig 7.) [5].

To build momentum, he must start bouncing, which typically causes Pistorius to start much

slower than a sprinter with biological legs. Hugh Herr, a professor at MIT who helped analyze

data on Pistorius and his Cheetah prosthetics, points out an additional disadvantage [5], [8].

According to Herr and his study of single-amputee sprinters, not having ankles causes amputees

to apply less force with their prosthetic leg than their biological leg, suggesting that the softness

of the prosthetic causes a force deficit [5], [9]. This is because without ankles, the leg’s ability to

stiffen is greatly reduced, which in turn limits the leg’s ability to hit the ground as forcefully as

able-bodied runners [8]. "It's like running on a springy mattress," he says [8], [9]. "The ankle is

an important joint for modulating whole leg stiffness, and it's been shown that a runner like

Pistorius doesn't have that dramatic leg stiffness change." Herr has also contends that the faster

leg swing times merely make up for this force deficit [5].

Still, others remain skeptical. “With the most generous assumptions, he still comes out seven

seconds ahead in the 400," Weyand said. "He's a below-average high school runner without those

limbs. A lot of people don't want to hear that. [5], [9]" Weyand added also that he doesn't dispute

that an ankle might be an advantage, but he says a mathematical equation can account for it:

speed (meters/second) = stride length (meters) X stride frequency (strides/second) [5].

"If having no ankle affects speed, it must do so through an effect on either stride length, stride

frequency or both," he says. It is suffice to say that there is no clear answer and that more

studies will inevitably be done on the issue. When asked if we will ever know all of the

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advantages and disadvantages of the Cheetah, Herr’s response was "It's going to take years and

years, and it may not be knowable [8]."

Looking to the Future of the Flex-Foot:

The Flex-Foot Cheetah design has remained very similar to its debut design in 1997 [4]. The

Cheetahs were made specifically for sprinting and because they lack a heel, they can be very

difficult to use when trying to simply stand or walk around [8]. "They're missing the heel section,

because sprinters are toe strikers," explains Richard Hirons, clinical specialist at Ossur [8]. "It

has the profile of a foot, but it's pointing downward, like you're standing on tiptoes." The user

may also experience soreness where the prosthetic meet the remaining part of the legs [8].

Pistorius wears a different, more comfortable pair of carbon fiber feet with heels for daily life

because of this. Perhaps future designs will look to diminish the differences between the two. "In

a sprint, you think of the different stages: the drive and acceleration from the blocks where you

start in a squat and bend forward in transition," Hirons said. "You have the normal human

anatomy working in one way, then in the run in a different way. The [current] prosthetic is

optimized one way or another. In the future, there may be a more refined design to minimize the

compromise [8]."

Conclusion:

While the advantages and disadvantages of the Flex-Foot Cheetah are still being debated,

interesting questions are being raised about how we want to govern sports.

“It's not our place [as researchers] to make the policy," Weyand said. "Those are hard decisions,

and it depends philosophically on what sport should be. On the one extreme, is it supposed to be

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biological and pure,” asked Weyand, “or on the other, will we be perfectly okay with it being a

gladiator sport and pharmaceutical freak show? [8], [9]”

Herr adds, “People have always thought the human body is the ideal. It’s not [8].” With this

notion in mind, maybe the question on sports policy is not about having purity versus having a

“freak show” as Weyand suggests, but rather at what point does enhancement need to stop?

It would seem that in the face of new technology, we as humans, as usual, have serious thinking

to do. New technology has always threatened to change the landscape of our world. However, it

hasn’t been until recently sports have been so profoundly impacted. Perhaps the Flex-Foot

Cheetah is just the tip of the iceberg. Andy Miah, chair in Ethics and Emerging Technologies at

the University of the West of Scotland, speculates that “Fifty years from now, we may not have

[the Olympics and Paralympics]—we may have only one set of performances that people

compete in, that reveal how capable they are at using their bodies in combination with

technology…” he says. “When I tune into the Olympics 50 years from now, I not only expect to

see different kinds of sports [than we have today], but you never know—I may even be

competing there [9].”

A future like that may be close or it may be lifetimes away, but for now most of us can agree that

the Cheetah is a truly exciting and amazing invention. The technology of the Cheetah gives those

without legs the opportunities that life, at some point, may not have afforded them. In turn, both

the abled and disabled are left inspired. Weyand expresses a similar sentiment. “What Oscar has

done represents for a lot of people an unwillingness to accept expectations others might impose

on you," Weyand says. "And that part is inspiring and makes you feel great about human nature

[5]."

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To see Oscar in action and a visual look on how his legs work, click the link below.

http://science360.gov/obj/video/d2783a15-2880-4c1d-b6b4-d795c129aae2/science-summer-olympics-strength-flexibility-oscar-pistorius

Citations:

1. P. Rosales. (2012, August 12). World record breakers at the 2012 Olympics [Online] http://sports.yahoo.com/photos/olympics-record-breakers-of-2012-1344773931-slideshow/

2. G. A. Davies (2007, May 09) My Sport: Oscar Pistorius [Online] http://www.telegraph.co.uk/sport/mysport/2312850/My-Sport-Oscar-Pistorius.html

3. (2007, January) Flex-Foot [Online] http://web.mit.edu/invent/iow/phillips.html4. (2012) Cheetah [Online] http://www.ossur.com/?PageID=134625. D. Epstein (2012, August 04) Fair or foul? Experts split over whether Pistorius has

advantage [Online] http://sportsillustrated.cnn.com/2012/olympics/2012/writers/david_epstein/08/03/oscar-pistorius-london-olympics/index.html

6. S. Magness (2010, August) How to Run: Running with Proper Biomechanics [Online] http://www.scienceofrunning.com/2010/08/how-to-run-running-with-proper.html

7. (2012) Carbon-fiber-reinforced polymer [Online] https://en.wikipedia.org/wiki/Carbon-fiber-reinforced_polymer#Sporting_goods

8. S. Eldred (2012, July 20) How Olympic ‘Blade Runner’ Sprints Without Feet [Online] http://news.discovery.com/adventure/oscar-pistorius-amputee-olympics-120720.html

9. J. Turbow (2012, August 03) Bigger, Faster, Stronger: Will Bionic Limbs Put the Olympics to Shame? [Online] http://www.wired.com/playbook/2012/08/next-gen-prosthetics-and-sports/all/

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