Blacks, on average, have more fast twitch muscle fibers and greater amounts of testosterone than whites.
Something as major as an additional pair of muscles would have required major evolutionary changes. Or a loss of a pair of muscles, since all humans evolved from a common ancestor. In order for black people to have "an extra muscle" from other human beings there would have to be fixed genetic differences between races.
Meaning a clear break
Indeed this seems like a speciation level difference to be frank, such as "Blacks" having a gene for X and no one else. This is absolutely clearly not the case. Not a shadow of a doubt.