Long Fall Boots
The Long Fall Boots, or simply known as the Boots, were worn by human Test Subjects during the testing of the Handheld Portal Device in the Aperture Science Enrichment Center. These boots were created as a means of ensuring that the Portal Devices would not inflict damage, as well as to resolve a "reproducible human error problem".
Taking place after the events of Portal, Chell's Advanced Knee Replacements were torn apart during the Portal 2: Lab Rat comic. As a result, the Party Escort Bot later fits her into the Long Fall Boots prior to her awakening in Portal 2.
The boots were designed to prevent human Test Subjects from injuring themselves when falling from great heights at any speed, even terminal velocity. Although the boots appear to be a safety precaution for the benefit of Test Subjects, the rationale behind their creation was by no means altruistic. In fact, the Long Fall Boots were created in order to protect the Handheld Portal Device from being destroyed when Test Subjects fell from extreme heights. Although Test Subjects were considered expendable, manufacturing Handheld Portal Devices required significant expenditure.
The Long Fall Boots appear to have a strengthening or tightening force around the legs down to the feet. This effect can be heard every time Chell lands onto a surface after falling from great velocities.
Despite appearing more logical and useful for its ability to shield entire feet and legs, it remains unknown as to why these boots were not implemented on Chell any sooner instead.