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The battle against doping in sport is a never ending one as anti doping authorities struggle to keep up with advances in technology and masking agents.
But where does that leave athletes who have had some kind of surgery or use a prosthesis that aids performance, such as the blades used by Oscar Pistorius?
Dr Alan J. Levinovitz from James Madison University's Department of Philosophy and Religion argues that the line is getting increasingly blurry and raises questions about what the word natural actually means when it comes to athletes.
And he says as biotechnology continues to advance it will become increasingly difficult to define "natural".
