Mechanics of Oscar Pistorius' running blades
Oscar Pistorius is a South African sprint runner with double below the knee amputations who competes in both able-bodied and T44 amputee athletics events. He is the T44 world record holder for the 200m and 400m events. Pistorius' eligibility to run in international able-bodied events is sanctioned by the International Association of Athletics Federations (IAAF).
Pistorius began running in 2004 after a rugby knee injury led to rehabilitation at the University of Pretoria's High Performance Centre with coach Ampie Louw. His first racing blades were fitted by South African prosthetist Francois Vanderwatt. Because he was unable to find suitable running blades in Pretoria, Vanderwatt ordered some to be made by a local engineer at Hanger Orthopedic Group. These quickly broke, and Vanderwatt referred Pistorius to American prosthetist and Paralympic sprinter Brian Frasure to be fitted for carbon-fibre blades by Icelandic company Össur.
Pistorius' participation in able-bodied international sprinting competitions in 2007 raised questions about his use of running blades, and the IAAF amended their rules to ban the use of "any technical device that incorporates springs, wheels or any other element that provides a user with an advantage over another athlete not using such a device." After initial studies, Pistorius was ruled ineligible for competitions under these IAAF rules. After further research was presented, the Court of Arbitration (CAS) ruled that his running prostheses were not shown to provide a net competitive advantage over biological legs. In 2012 Pistorius qualified for and competed in both the 2012 Olympic Games and the 2012 Paralympic Games using his running blades, becoming the first amputee sprinter to run in the Olympic Games.
Pistorius' athletics prostheses
Oscar Pistorius races with Flex-Foot Cheetah carbon fibre running blades. These are transtibial prostheses, meaning they replace legs and feet that are amputated below the knee (BK). Flex-Foot Cheetah blades were developed by medical engineer Van Phillips who incorporated Flex-Foot, Inc., in 1984, and in 2000 sold the company to Össur which now (in 2012) manufactures the blades. Unlike previous models of prosthetic legs and feet, the Flex-Foot Cheetah was designed to store kinetic energy like a spring, allowing the wearer to jump and run effectively.
Carbon fibre is actually a carbon-fiber-reinforced polymer, and is a strong, light-weight material used in a number of applications, including sporting goods like baseball bats, car parts, helmets, sailboats, bicycles and other equipment where rigidity and high strength-to-weight ratio is important. The polymer used for this equipment is normally epoxy, but other polymers are also used, depending on the application, and other reinforcing fibres may also be included. In the blade manufacturing process, sheets of impregnated material are cut into square sheets and pressed onto a form to produce the final shape. From 30 to 90 sheets may be layered, depending on the expected weight of the athlete, and the mold is then autoclaved to fuse the sheets into a solid plate. This method reduces air bubbles that can cause breaks. Once the result is cooled, it is cut into the shape of the blades. The finished blade is bolted to a carbon fibre socket that is an intimate fit to each of Pistorius' legs. These are custom made and make up the bulk of the total cost, along with the assessment and setting up of the finished prostheses. Each limb costs between $15–18,000 USD.
Pistorius has been using the same Össur blades since 2004. He was born without fibulae and with malformed feet, and his legs were amputated about halfway between knee and ankle so he could wear prosthetic legs. He wears socks and pads which are visible above the sockets to reduce chafing and to prevent blisters, and the sockets have straps in the front that can be tightened to make the prosthesis fit more snugly.
Pistorius uses spike pads made by Nike on the blades. Before development of the pads, his spikes were changed by roughing up the surface and applying over-the-counter spikes by hand, but the results using this method were inconsistent. Nike conducted research in Össur’s Iceland lab using a pressure-sensitive treadmill and film at 500 fps to measure the blade strike, and produced a spike pad which includes a midsole of two machine-molded pieces of foam of different densities to cushion impact, with a carbon fibre plate on the bottom. The developers attached the pad with contact cement, which can be quickly removed with the application of heat when the spike pad needs to be changed.
Because of the curved design, Flex-Foot Cheetah blades have to be slightly longer than a runner's biological leg and foot would be. The blades replace the hinge of an ankle with elastic compression that bends and releases the blade with every stride, so the uncompressed blade leaves the user standing on tiptoe. They are designed to move forward, so have no heel support in the back. According to Josh McHugh of Wired Magazine, "The Cheetahs seem to bounce of their own accord. It’s impossible to stand still on them, and difficult to move slowly. Once they get going, Cheetahs are extremely hard to control."
How the blades work
In 2007 Pistorius applied to run in able-bodied track meets. He was at first accepted, but questions quickly arose about whether the blades give him an unfair advantage. After initial research showed the blades did provide an advantage, the International Association of Athletics Federations (IAAF) changed their rules to ban the use of technical devices that provide an advantage and ruled him ineligible to compete. Pistorius challenged the ruling with additional research and was reinstated by the Court of Arbitration for Sport (CAS) in 2008, meaning that he can continue to run in able-bodied meets as long as he uses the equipment that was studied in the research.
Pistorius' performance in the early able-bodied races raised questions because of two major concerns: his pattern of running the races and his leg-swing times. Most sprinters spring out of the blocks with their fastest time and slow down as the race progresses, but Pistorius ran a "negative split," starting slowly and building up speed in the last half of the race (though he no longer uses this pattern). His average time was also less in the 400m race when compared to other runners than in the 200m. Controversy about the use of the blades persists, but the research provided considerable information on how they work in application, and other research is expected to follow.
Able-bodied sprinters have calves and ankles that return and amplify the energy supplied by their hips and knees, while Pistorius compensates with additional work because he does not have calves and ankles with their associated tendons and muscles. An analysis published by Engineering & Technology magazine estimates that in using the blades, Pistorius must generate twice the power from his gluteal and quadriceps muscles that a normal sprinter would. Other sources also credit core abdominal muscles and a faster arm swing. His trainer estimates that about 85 percent of his power comes from his hips and the rest from his knees. This results in a gait that waddles slightly, as Pistorius swings his upper body to balance the springing action of the blades. The blades compress under his weight, then release as he moves forward, providing forward thrust from the tips as they return to their molded shape. As they spring off, he swings them slightly out to the side and throws them forward for the next stride.
Pistorius is always slow in starting a race because the flexible blades do not provide thrust out of the blocks. Pistorius must begin from an awkward position, swing his leg to the outside and pop straight up from the blocks to begin running, when the preferred method is to push off with horizontal force. For the first 30 meters of a race, he keeps his head down and takes short, quick strides. As he establishes a rhythm, he can raise his head and increase his speed. While some runners jog up and down, losing energy, Pistorius directs energy forward, looking somewhat like he is rolling on wheels. He also compensates for the adjustments ankles make on the turns, breaking the curves into short, straight lines. According to his coach Ampie Louw, Pistorius may be able to use the inward lean to generate force and come out of a turn going faster.
To resolve questions about the blades, Pistorius was asked to take part in a series of scientific tests in November 2007 at the German Sport University Cologne with Professor of Biomechanics Peter Brüggemann and IAAF technical expert Elio Locatelli. After two days of tests, Brüggemann reported that Pistorius used about 25 percent less energy expenditure than able-bodied athletes once he achieved a given speed. The study also found that he showed major differences in sprint mechanics, with significantly different maximum vertical ground return forces, and that the positive work or returned energy was close to three times higher than that of a human ankle. The energy loss in the blade during stance phase when the foot was on the ground was measured as 9.3 percent, while that of normal ankle joint was measured at 42.4 percent, showing a difference of more than 30 percent. Brüggemann's analysis stated that the blades allowed lower energy consumption at the same speed, and that the energy loss in the blade is significantly less than in a human ankle at maximum speed. In December of that year, Brüggemann stated to Die Welt newspaper that Pistorius "has considerable advantages over athletes without prosthetic limbs who were tested by us. It was more than just a few percentage points. I did not expect it to be so clear." The study was published in 2008 in Sports Technology, and later researchers stated that the analysis "did not take enough variables into consideration".
Weyand, et al. study
In 2008 a team of seven researchers conducted tests at Rice University, including Peter Weyand, Hugh Herr, Rodger Kram, Matthew Bundle and Alena Grabowski. The team collected metabolic and mechanical data by indirect calorimetry and ground reaction force measurements on Pistorius' performance during constant-speed, level treadmill running, and found that the energy usage was 3.8 percent lower than average values for elite able-bodied distance runners, 6.7 percent lower than for average distance runners and 17 percent lower than for able-bodied 400m sprint runners. At sprinting speeds of 8.0, 9.0 and 10.0 m/s, Pistorius produced longer foot to ground contact times, shorter leg swing times, and lower average vertical forces than able bodied sprinters. The team concluded that running on the blades appears to be physiologically similar but mechanically different from running with biological legs. The study was published several months later in the Journal of Applied Physiology. The inconsistencies between the finding of this study and the Brüggemann study were attributed to differences in study methodology.
In the study, the blades were found to have an elastic energy return of about 92 percent, whereas biological legs provide between a 93 and 95 percent return. Grabowski also stated that the prostheses reduce the amount of force Pistorius can apply to the ground when he runs, reducing his ability to propel himself forward. The shape of the blade foot is a longer lever than the human foot, providing a contact point further away from the axis of rotation than a real foot. This would allow greater torque generation when an identical amount of force is applied, but because of the springy quality of the blades, Pistorius is unable to exert the same force as an able-bodied runner during push off from the ground. Kram also stated that Pistorius' "rate of energy consumption was lower than an average person but comparable to other high-caliber athletes."
The lightness and rigidity of the blade compared to muscle and bone may allow blade runners to swing their legs faster than able-bodied runners. In comments on the article, Peter Weyand and biomechanist Matthew Bundle noted that the study found that Pistorius re-positioned his legs 15.7 percent faster than most world record sprinters, allowing for a 15 to 30 percent increase in sprint speed.
Grabowski, et al. study
In 2009 a research team including Alena Grabowski, Rodger Kram and Hugh Herr conducted a follow-up study of single amputees with running blades which was published in Biology Letters. Each of six amputees' affected leg performance was compared against that of their biological leg. The team measured leg swing times and force applied to the running surface on a high-speed treadmill at the Biomechanics Laboratory of the Orthopedic Specialty Hospital, and also studied video of sprint runners from the Olympics and Paralympics. They found no difference in leg swing times at different speeds, and recorded leg swing times similar to that of able-bodied sprinters. They also found that single running blades reduced the foot to ground force production of the tested runners by an average of 9 percent. Because force production is generally considered the most significant factor in running speed, the researchers concluded that this reduction in force limited the sprinters' top speed. Grabowski also found that amputees typically increased their leg swing times to compensate for the lack of force.
Discussion continues about the relative advantage or disadvantage of using the blades. Researchers and analysts also point out that the research studies are done on level, stationary treadmills, and do not measure performance from starting blocks or on actual curved tracks. They also do not take into the account differences in physiology between amputees and non-amputees, who have such factors as musculature, blade height and weight and differences in blood circulation patterns due to the history of their limb loss.
A controversy over the effects of running blade length arose at the 2012 Paralympic Games, as Brazilian runner Alan Oliveira and USA runner Blake Leeper changed to longer running blades within a few months before the 2012 Paralympic Games, leading to marked improvement in their running times. Pistorius complained after the 200m race that the blades provided artificially lengthened running strides, which would be an infringement of the IPC rules, regardless of that the blades were within the allowable height limits for the athletes concerned. His complaint was supported by single-leg runners including Jerome Singleton and Jack Swift, who called for the T43 double blade and T44 single blade classes to be separated in future events, as single blade runners were unable to adjust the height of the prostheses, and must always match the length of their biological leg with a running blade.
The improvement in running time and the wide broadcast of the race results provided a public demonstration of how the blade length affects performance. Pistorius' stride length was actually 9 percent longer (2.2m vs 2.0m), but Oliveira took more strides (99 vs 92). The combination of stride length and stride rate led to a clearly unusual performance with the longer blades. Pistorius' management issued a statement saying that Pistorius is always 1.84 meters tall, regardless of what prostheses he wears, and that the decision to maintain this height for his running blades was an issue of fairness.
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