The central knots in most cases give rise to excess surface brightness. To quantify the strength of the starburst the excess light may be integrated and compared to the underlying galaxy light. Given, that it must have a low mass to light ratio, this suggests that the starburst contains only a minor fraction of the integrated stellar mass, and that the subsequent fading in luminosity will be very moderate. Of course, some of these galaxies may already have passed their SFR peak and be in the process of fading. However, the amount of central excess depends on how luminosity profiles were constructed and to what depth the profiles are fitted, and moreover depends on an a priori assumption of the true shape of the underlying galaxy.
Colour profiles can here yield useful additional information.