By Tabitha S – U6
In this essay I aim to determine whether athletes utilising advanced sports prosthetics have an advantage over able bodied athletes. Initially I started researching into the general topic of prosthetics, which greatly interested me because it applies numerous scientific disciplines to make a very meaningful and positive difference to the lives of amputees. I discovered that the ethical implications of using prosthetics are incredibly important, especially as advancements in prosthetic design propel the performance of such devices far beyond what we may have considered possible even ten years ago.With this in mind, I decided to focus my research on the controversial issue of sports prosthetics for disabled athletes; a topic that was thrown into the media spotlight after Oscar Pistorius was banned from racing against able bodied athletes using his prosthetic blades (called Flex Foot Cheetahs) in 2007, allegedly because they gave him an unfair advantage.  After an appeal and consequent further study of his blades it was decided that he could compete as he had no net advantage, and in the 2011 world championships he became the first amputee to win a medal whilst competing against able bodied athletes.  I have focused the majority of my research on how his blades enabled Pistorius to do this, and on whether or not the decision that Pistorius, or any other para-athlete, has no net advantage due to their prosthetics is correct. Through the course of this essay I intend to prove that although we cannot know for certain, it is highly likely that they do have an advantage due to the way in which sports prosthetics, including Flex Foot Cheetahs, store elastic potential energy and use it to propel a sprinter forwards.
Initially, the independent 2007 study commissioned by the IAAF (the athletics governing body which regulates the eligibility of competitive athletes) established that Pistorius’s Cheetahs broke rule 144.2, which prohibits the “use of any technical device incorporating springs, wheels or any other element that provides the user with an advantage over another athlete not using such a device”. The study compared Pistorius to five able bodied athletes achieving similar levels of performance at 400m, and showed he had a clear advantage.  
However, the study had several key issues which I think prevent it from being a reliable measure of the supposed advantage of sports prosthetics. Firstly, it only studied the athletes running at constant speed; it failed to account for the acceleration phase of the race and therefore didn’t measure the effect of the prosthetics over the whole race. Furthermore, Pistorius later commissioned independent researchers who produced contradictory results regarding oxygen uptake and energy output, which they found to be equal to that of a comparable able bodied athlete  unlike the original study. The fact that no other para-athlete has ever reached Pistorius’s level of sprinting success is also important; he might simply be extraordinary. Overall, I feel that these issues discredit the legitimacy of the initial study. We cannot use it to say that Pistorius definitely has an advantage, nor can we use it to say that he doesn’t. Any successful comparative experiment between Pistorius and an able bodied athlete should, in my opinion, be a more extensive long term study measuring the effect of a prosthetic over an extended period of time to account for fluctuations in fitness and performance, and holistic challenges that arise from using blades.
However, studying the biomechanics of how a sprinter runs efficiently and comparing it to an amputee could help us to distinguish whether sports prosthetics give the user an advantage. Figure 1 in the appendix shows the sprinting gait cycle, which begins when one foot comes into contact with the ground and ends when the same foot touches the ground again. Stance phase is when the foot is in contact with the ground and “toe off” marks the beginning of swing phase, when the foot is off the ground. Before and after swing phase is float phase, when neither foot is touching the ground. Sprinters toe off as early as 22% of the way into the gait cycle , so the majority of time is spent in the swing phase. This is advantageous for amputees using flex foot cheetahs as each blade is extremely light, weighing less than two pounds .
Potential and kinetic energy phases are in sync when sprinting, peaking during swing phase and dropping as stance phase begins.  When the foot touches the ground kinetic energy is lost, and as the centre of mass falls towards the ground due to bent legs, potential energy is lost, leading to large fluctuations of energy in and out of the system, which need to be minimised for an efficient sprint that requires as little work as possible. This is facilitated by the storage of elastic potential energy through the stretch of tendons and muscles within the body, which act as system of biological springs. However, this system is not efficient and large amounts of energy are lost as heat due to respiration of muscle cells.
Here, I would argue, is where a Flex Foot Cheetah, made of carbon fibre reinforced polymer, might give the user an advantage. Fibres of carbon entwined with a binding matrix allow it to compress and store energy during the stance phase, similar to how tendons operate in the leg. In swing phase, the prosthetic returns to its original shape which releases elastic potential energy, propelling the runner forward.   Furthermore, the carbon fibre layering is optimised through computer analysis to ensure that the deflections of the carbon fibre are proportional to the user’s weight and impact level, meaning the minimum amount of energy is lost and the maximum horizontal propulsion is achieved for each gait cycle.  Arguably, no human leg could be as efficient as a designed personalised carbon fibre alternative. In the constant phase stage of a 400m race, it is highly likely that an amputee using a flex foot cheetah would have an advantage over an able bodied athlete.
However, when a sprinter accelerates, the pelvis and trunk tilt further forward, meaning that the centre of mass moves vertically downwards and further forward than the contact point between the toe and the ground (see Fig. 2 in the appendices).This occurs in order to maintain a ground reaction force that acts in a non-vertical position. The ground reaction force is the equal and opposite force that the ground exerts on the runner in response to the force that the runner exerts on the ground. A non-vertical reaction force will have a horizontal component which allows forward acceleration. As a sprinter’s centre of mass loses vertical height, the sprinter loses gravitational potential energy. Sprinters aim to begin their races as low as possible by using start blocks, in order to minimise this energy loss.  It is clear that here athletes using prosthetics have a disadvantage. Prosthetics do not allow users to begin at starting blocks, so they must start the race with a higher centre of mass. They therefore lose more gravitational potential energy, which means that acceleration at the same rate as an able bodied sprinter requires more energy.
To conclude, I have shown that amputees will have a considerable disadvantage during the acceleration phase of a race due to their inability to minimise loss in potential energy. Until sports prosthetic can successfully mimic the action of the ankle, para-athletes will continue to be disadvantaged in this area. However, I believe I have shown that they will have a sizeable advantage as soon as they reach constant velocity given the way in which the carbon fibre layering in the prosthetic is optimised to achieve the most efficient conversion of elastic potential energy to kinetic energy. This means that the prosthetic can reduce the energy loss during each gait cycle. The user can therefore sustain higher speeds with a smaller amount of work compared to an able bodied athlete. It is worth noting that even fractions of a second will make a huge difference; a small time advantage could change the entire race result. However, we cannot be sure that the advantage during constant speed is enough to qualify as an overall advantage. This would depend on each individual athlete, on their ratio of acceleration to constant speed during a race, and on their individual technique for starting without blocks. Furthermore, the exact composition of the carbon fibre used for flex foot cheetahs is not public knowledge, and further study of the material would be required to understand exactly how it stores elastic potential energy and to what extent it converts elastic energy to kinetic energy more efficiently than a biological leg. Whilst I cannot answer my question for certain, I would suggest that far more caution be exercised when allowing athletes using prostheses to race with able bodied athletes given their advantage at constant speed, especially in the future when technology might propel human achievement far beyond what we consider natural.
|||“Internation Association of Athletics Federations,” [Online]. Available: https://www.iaaf.org/news/news/oscar-pistorius-independent-scientific-stud-1.|
|||[Online]. Available: https://en.wikipedia.org/wiki/Oscar_Pistorius#Non-disabled_sports_events.|
|||“IAAS Rulebook,” [Online]. Available: https://www.iaaf.org/about-iaaf/documents/rules-regulations.|
|||P. J. Zettler, “Is it cheating to use cheetahs?,” Boston University International Law Journal, vol. 27:367, pp. 372-374, 2009.|
|||P. J. Zettler, “Is it cheating to use cheetahs?,” Boston University Law Journal, vol. 27:367, pp. 374-378, 2009.|
|||T. F. Novacheck, “The Biomechanics of Running,” Gait and Posture, p. 79, 1998.|
|||“Ossur Prosthetics,” [Online]. Available: https://www.ossur.co.uk/prosthetic-solutions/products/sport-solutions/cheetah.|
|||T. F. Novacheck, “The biomechanics of running,” Gait and Posture, 1998.|
|||B. McCarthy, “Flex-Foot Cheetah,” Biomedical Engineering, University of Rhode Island, 2011.|