In testing the muscular strength in the south African natives the writer was somewhat surprised to find that these negroes gave rather smaller figures than one could expect with regard to their generally fine physiques.Strength has a large CNS component.
[. . .]
the average strength is 29.5 for the right hand in men and 23.7 for the same hand in women. These figures, so far as the males are concerned, are decidedly behind those of whites. Hrdlicka,13 in normal “ Old Americans,” using same type of instrument and same method, found the pressure strength in the right hand to average 41.8 kg. in the males and 23.2 kg. in the females.
[. . .]
These differences are noteworthy. [. . .] Every student carefully observing the everyday life of intelligent people will surely have noticed the fact that their physical output of work is often remarkable even with relatively poor muscles. This and observations such as those above render it probable that, other factors being equal, the more intelligence there is, the more physical as well as other efficiency.
[V. Suk. Anthropological and physiological observations on the negroes of Natal and Zululand. American Journal of Physical Anthropology. Volume 10 Issue 1, Pages 31 - 64 1927]
The central nervous system (CNS) is of paramount importance in the exertion and development of muscular strength. Muscular strength is determined not only by the quantity of involved muscle mass but also by the extent to which individual fibers in a muscle are activated (by intramuscular coordination).There's a fair amount of evidence whites average greater strength per unit of muscular cross section than blacks.
[Vladimir M. Zatsiorsky and William J. Kraemer. Science and practice of strength training.]
Among young men in the U.S., blacks are certainly leaner on average than whites. They probably average slightly greater muscle mass -- though, I don't know whether or not the difference would remain after controlling for higher levels of physical activity and greater energy intake in black males. The differences in muscle mass are also not of sufficient magnitude to explain the awe in which certain  internet posters hold black male physiques.
 Usually, it seems, "ethnic" urban types, as I suspect guywhite is. See also Peter "ironrailsandweights" Rosa, Steve Sailer, and too many Jews to count.