What makes the 9-second barrier so hard to break for 100m sprinters?

[Alex Mercer]

What limits the human body to running above 9 seconds and/or not even faster? I once saw a documentary about ancient aboriginals who lived in Australia, some guy said based on the footprints left behind he said that the aboriginal could run faster than Usain Bolt. Makes me wonder about whether people out there could exist that are faster than Usain Bolt or more generally what kind of people are hidden away that have hidden talents. But basically my question at the top!

 

Thanks in advance

Can't edit my answer for some reason, just has a small blank box that doesn't open bigger with my original post. 

Reason for edit: I mean running faster than 9 seconds for the 100m sprint. 

Woops I edited by mistake I re read and my initial post was correct. Excuse me 😛

[CharonY]

There are a couple of papers that describe the theoretical biophysical limitations. See here and most likely follow-ups can illuminate on further details.