How were infantry created in the early Medieval Ages? How did this change as a middle class began to emerge? Is there any major difference between the "Muslim" world and the "Christian" one?

I wonder if you could elaborate on what you mean by the 'creation' of infantry? I'll assume you don't mean it literally (give a man a weapon but don't give him a horse!) Are you looking for an answer about recruitment? Which social classes different types or troops were drawn from? Or do you mean something different? This information should help our answerers craft an answer that will suit the information you're looking for.