  Many observed globular clusters (GCs) seem to show a central overabundance of
mass whose nature has not yet fully understood. Indeed, it is not clear whether
it is due to a central intermediate mass black hole (IMBH) or to a massive
stellar system (MSS) composed of mass segregated stars. In this contribution we
present a semi-analytic approach to the problem complemented by 12 $N$-body
simulations in which we followed the formation of MSSs in GCs with masses up to
$3\times 10^5$ \Ms. Some implications for the formation of IMBHs and
gravitational waves emission are discussed in perspective of a future work.
