U.S. Olympic legend Michael Johnson believes hard work only accounted for some of his record-breaking athleticism ... the rest can be chalked up to the fact that his descendants were slaves. 

Johnson -- one of the greatest Olympic sprinters of all time -- was talking to Sally Beck of the Daily Mail -- when he was asked about his world class athletic talent. 

Johnson explained, "All my life I believed I became an athlete through my own determination, but it's impossible to think that being descended from slaves hasn't left an imprint through the generations."

He added, "Difficult as it was to hear, slavery has benefited descendants like me -- I believe there is a superior athletic gene in us."

"Over the last few years, athletes of...