an article of apparel for a human being , designed or arranged to be worn around an appendage such as an arm or leg , and designed or intended to resist impact . the article contains a material in the form of a spongy , elastic polymer engineered to absorb energy , shock , vibration and impact . the article of apparel can be used to protect hard , bony areas of the human body such as elbows , knees and ankles . the article will be of use to persons engaged in sports involving bats , sticks , mallets , pucks and hard balls , as well as to cyclists , and anyone whose occupation requires the knees or elbows to come into contact with hard surfaces .