If it is in fact true that black people have different muscle structure it is probably due to thousands/ millions of years of evolution…---Additionally, some cross-sectional studies have found that, as a population, African-Americans have a greater density of 'fast-twitch' muscle fibers within the muscle bodies. This sort of muscular morphology would also lend to a 'leaner' muscle body (with less fat between the muscle cells)… Anthropology teaches that blacks actually do have longer and stronger long bones in the body, ie femur, humerus etc. Therefore, there are more muscle fibres. The cause is most likely due to genetics and environmental factors.
(http://wiki.answers.com/Q/Why_is_the_muscle_structur... )

These data suggest that sedentary male Black individuals are, in terms of skeletal muscle characteristics, well endowed for sport events of short duration.
(http://jap.physiology.org/cgi/content/abstract/61/5/... )

to motivate.