Walkers are also available in other sizes such as pediatric (for children) or bariatric (for obese persons). Modern walkers are height adjustable and should be set at a height that is comfortable for the user, but will allow the user to maintain a slight bend in their arms. This bend is needed to allow for proper blood circulation through the arms as the walker is used. The front two legs of the walker may or may not have wheels attached, depending on the strength and abilities of the person using it. It is also common to see caster wheels or glides on the back legs of a walker with wheels on the front; additionally felt products adapted to glide with the legs such as adhesive feet, along with tennis balls with holes cut into them to place them onto the legs are also utilized on surfaces such as hardwood, epoxy and linoleum flooring common to institutions.