The thoroughbred racehorse is one of the greatest athletes on the planet, galloping with incredible speed and stamina for such a large animal. It is the result of unnatural selection, and exists on a knife edge between glory and catastrophic failure. The team explore how this animal has been biologically engineered for speed. They dissect an elite racehorse to reveal the extraordinary spring system that propels it to 45mph, its super-sized organs and built-in turbo-booster. Simon Watt visits the top breeding centre in Europe to find out how to produce a champion; and Mark Evans investigates the science behind their phenomenal performance and their vulnerability to injury