What degree should I pursue if I want to design athletic shoes?

Biomedical engineering is a good degree because you can use it to find various jobs besides designing athletic shoes. There are not too many jobs available in such a specialized field. If you think of a better design than is now on the market, make a prototype on your own time, patent it and try to sell it to a footwear company. Make sure that your day job does not have any claim to patents for work you do on your own outside of their facility before you start to work for them. This is called contract negotiation.