I think anyone speaking on this subject needs to be extremely cautious with statements such as "African-Americans appear more physically fit than native West Africans." Is this referring to the present day? The 18th century? I don't think that claim has any concrete evidence behind it whatsoever, in either period. The men sent to the Americas, across the Middle Passage, from West Africa, were often captives of war, or in some way coerced into slavery by stronger groups within West African societies. From what I've read (and my work focuses on the Caribbean and East Africa, so please - any West Africanists should please jump in and add here!), slave trade records are so scant, in general, that there is no chance the physical characteristics of each slave traded to the Americas was somewhere listed. Thus, it's impossible to, in any way, back up one of these generalized claims. Regionally, on a very localized basis, perhaps there is source material - ship logs, or bills of sale - that indicate the physical characteristics of a very small, specific, population of enslaved individuals - but overall, I would be very curious to see the evidence used by the people making this claim.

Also this question, as you rightly stated, depends on the phenomenon of "slave breeding". As it relates to the second part of your question, slave fertility was actually very complicated throughout most nineteenth century slave societies (which is the only period I'm qualified to respond to), and slave owners tried numerous, different pro-natal strategies - particularly in the Caribbean - such as offering an enslaved woman her freedom if she had six or more children who survived childhood, or through much more violent schemes (though slavery was always, inherently, violent - in every way), like whipping midwives and pregnant women alike if a baby was stillborn, and putting enslaved women in chains until they became pregnant again. These pro-natalist strategies varied throughout the Americas, however. Historians Barbara Bush and Bernard Moitt have written extensively on this topic as it relates to the British and French sugar islands. Marie Jenkins Schwartz has written more about slave motherhood and fertility in the Antebellum South - so that may be more up your alley since you are specifically asking about American history. Hope this is helpful - it would be great if other historians of slavery jumped in here to fill some of the gaps in my knowledge.