In the interview posted on this page, the author mentions bellybuttons. The original article is not based on bellybuttons, it is a theoretical paper using a model of animal locomotion that explains how a higher center of gravity would increase running speed and decrease swim speed. Blacks have a higher relative center of gravity than whites. In top competition, blacks are faster runners and slower swimmers than whites.
So the authors use their model to explain how racial differences in running/swimming may occur.
They did not actually perform direct correlations using bellybuttons, center of gravity, running speed, or swim speed.
If different ratios of fast-twitch to slow-twitch muscle fibres is important for running than swimming, then that may be another avenue of research.
Bodyfat would help boyancy, but not horizontal swimspeed.
Blacks have a higher relative center of gravity than whites.
In top competition, blacks are faster runners and slower swimmers than whites.
So the authors use their model to explain how racial differences in running/swimming may occur.
They did not actually perform direct correlations using bellybuttons, center of gravity, running speed, or swim speed.
If different ratios of fast-twitch to slow-twitch muscle fibres is important for running than swimming, then that may be another avenue of research.
Bodyfat would help boyancy, but not horizontal swimspeed.