Photoshop help

[hellogreg]

I am trying to blur a back ground in photoshop. The image is of a person standing on a cat walk during a fashion show, can some one explain to me how I can blur the background but keep the section of the cat walk below their feet in focus ?

Many thanks Adrian ......

 

[LC2]

Have a look at this thread regarding masks, but rather than desaturating, use lens blur on the background.
http://www.talkphotography.co.uk/threads/layer-masks-a-beginners-guide.37165/