well, there are a few ways to do this.
The first is good old fashion Rotoscoping. Export your video into an image sequence and edit out all the color except the shoes frame by frame. This is long and tedious but gives maximum control.
A much easer/faster way, would be to import your footage to after effects, and use: Effect>Color Correction>Leave color. This method may glitch out quite abit if your lighting isn't perfect.
I havn't gotten good results with this yet, but you could screw around with: Effects>Keying>Extract. Set the channel to Red and play with the black/white points. I'm only mentioning this because I havn't played with it alot and maybe with some luck you'll have better results than I.
The last method, and supposivly the best, would be to buy the plugin "Silhouette Roto." I havn't used it, so I can't say how well it works but it's a program made to use motion tracking on masks. This would be your best option if you have the money.
Good luck with your movie