Mark Floro
Writ 340
Illumin Paper
The 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 carbonfiber 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
1
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.
Figure 1: Oscar Pistorius sprinting using his
Cheetahs.
http://www.telegraph.co.uk/sport/olympics/news/9
515233/Paralympics-2012-Oscar-Pistorius-makeshis-entrance.html
The History Behind the Blades
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
Figure 2: The initial design for the
Cheetah feet by Van Phillips.
http://web.mit.edu/invent/iow/philli
ps.html
University, he began the initial conceptualizations of a
2
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
Figure 3: Today’s J-shape design
and the current model of the FlexFoot Cheetah.
artificial racing limbs [3]. Today’s design has now
progressed into a J-shape and is known as the Flex-Foot
http://www.ossur.com/?PageID=13
462
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
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
3
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
Figure 4: Graph showing the relationship
the force being applied versus the foot is
touched to the ground.
http://barefootrunning.fas.harvard.edu/4
BiomechanicsofFootStrike.html
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 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].”
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
Figure 5: A simplistic look at how the FlexFoot Cheetah works.
an able-bodied runner replicating both the stance and
http://blog.newscom.com/?p=10759
swing phases of running [4].”
What It’s Made Of
The Flex-Foot Cheetah is made of a carbon-fiber-reinforced polymer [4]. Carbon-fiberreinforced polymer, more commonly known as carbon fiber, is a material that has many
applications 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
Figure 6: Picture that shows the two
parts of carbon-fiber-reinforced
polymer.
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
http://stormdancer.huntress.org/crpfa
q.htm
durable, as well as flexible but yet still rigid. The foot’s ability
5
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 intactlimbed 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
come drawbacks. Sports Illustrated’s David Epstein points
out one disadvantage of using the Flex-Foot Cheetah; the fact
Figure 7: Oscar’s stance as the race begins.
that Pistorius must stand up straight at the start of a race
http://www.usatoday.com/sports/olympics/lo
ndon/track/story/2012-08-04/oscar-pistoriousreaches-400-semifinals/56768470/1
(Fig 7.) [5]. To build momentum, he must start bouncing,
6
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
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]."
7
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
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]”
8
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]."
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-olympicsstrength-flexibility-oscar-pistorius
9
Citations:
1. P. Rosales. (2012, August 12). World record breakers at the 2012 Olympics [Online]
http://sports.yahoo.com/photos/olympics-record-breakers-of-2012-1344773931slideshow/
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.html
4. (2012) Cheetah [Online] http://www.ossur.com/?PageID=13462
5. 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/oscarpistorius-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/Carbonfiber-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-genprosthetics-and-sports/all/
10