The purpose of a footrest is to allow stability when it is otherwise not attainable in a good biomechanical and physiological manner. It brings the floor up to the level of the feet. It needs to be stable.

This article looks at the good and not so good aspects of using a footrest. I use one at work and find it generally a good thing. It's either that or get my desk lowered by 2 inches.
Footrests

Hi Virginia:

I am noticing a lot of organizations are now providing footrests for employees, but for those times when mine did not - used the famous phone books(lol).

I often wondered, even before I saw your post, why my feet felt a whole lot better at the end of my work day.

Thanks as usual for the info - much APPRECIATED

There are two problems with phone books. First, you have to have one the right height. Second, they are rarely large enough to fit your feet and allow room for natural movement. Third, they can become slippery pretty quickly as the binding weakens.

I've found that wrapping them in brown paper and maksing or strapping tape is a help. That way you can even stack two small ones together, or a large and a small. it also retards the binding degeneration.

I don't have a solution to the size issue, however.

I like the inclined foot rests, they are more comfortable for the knees.