The World Federation One Stop Fiqh

Ruling 182

Based on obligatory precaution, if the palms or knees of someone who moves around on his hands and knees become impure, they do not  become pure as a result of him moving around on them. The same applies to the bottom of a walking stick, the bottom of an artificial leg, the shoe of a quadruped animal, the wheel of a motorcycle or car, and similar things.