This is a great question. If we look a sprinter like Usain Bolt as an example we will get an idea. A sprinter has to have muscle that can produce force very quickly. This means they are able to recruit their fast twitch muscle fibres very quickly. They have more fibre per each nerve than most people. A sprinter will also be able to store and release more energy needed for these fast movements in their muscle cells, called Phosphocreatine. Also to be fast you need long limbs. Speed is simply stride length x stride frequency.