Recently a pupil told me he was having back problems which he put down to the traditional footstool. I think this can particularly be an issue with newcomers to classical guitar playing when, coupled with other technical considerations, tension can easily be introduced which can make the whole playing experience uncomfortable.

Fortunately there are alternatives to footstools which allow the instrument to be played with both feet on the floor, as it were, thereby setting up a more natural seating position which should eliminate much of the tension created when using a footstool. Almost all of them attach to the side of the guitar with suction cups and straddle the left leg (for right-handed players), maintaining a correct holding position.

Probably the most popular model is the Ergoplay Gitano Professional model support but there are variations available including the Tappert and the more adjustable Troster models.

(to see an Ergoplay support in use visit: http://youtu.be/MVfFhcfnJec 
and to see some of the most popular models visit: http://youtu.be/OKAGDwNipgc)

Prices start at around £24.00 in the UK or US$37.00 in the US (excluding delivery). You can do a Google search for sales outlets or try spanishguitar.com in the UK or bigmo.com in the US. Some suppliers, like bigmo.com, will ship worldwide.