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"Ewww, what's wrong with your legs?"
GLaDOS' Curiosity Core[src]

The Advanced Knee Replacement is a technique used by Aperture Science to prevent Test Subjects from injury from huge drops by lessening the impact.


  • The device consists of two prostheses completely or partially replacing the knee, partially tied to the Test Subject's calves, and prolonged with two flexible curved pieces of metal going under the Test Subject's heels, acting as springs.
  • In "Orientation Video no. 1", the diagram about jumping from high ledges suggests Advanced Knee Replacement is not commonly used by Test Subjects and might have been used exclusively for Chell.

Behind the scenes[]

Assassin springs

Detail of the Combine Assassin's heel springs.

  • The Advanced Knee Replacement was added to Portal due to disbelief by playtesters that Chell could survive the drops she was subject to, particularly those that involved falling recursively through portals.[3]
  • According to Matt Charlesworth, the team was not sure whether to keep the Advanced Knee Replacement in Portal 2. He stated that some team members were attached to it and that some were not, so they experimented what to do with them, or how to replace them,[4] until they decided to replace it with a variant, the Long Fall Boot.


  • The sound files for the Advanced Knee Replacement footsteps use the name "future shoes".



Portal 2[]

List of appearances[]