  The aim of this paper is to determine the initial rotational velocities
required on the ZAMS for single stars to reach the critical velocity, sometimes
called the break-up velocity, during the Main-Sequence (MS) phase. Some useful
relations between $\Omega/\Omega_{\rm crit}$, $\upsilon/\upsilon_{\rm crit}$
($\upsilon$ is the velocity at the equator), the moments of inertia, the
angular momenta, the kinetic energy in the rotation and various other basic
physical quantities are obtained.
