 The blades are transtibial prostheses, meaning they replace legs and feet that are amputated below the knee (BK).They 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.They are designed to store kinetic energy like a spring, allowing the wearer to jump and run effectively.