Many people seem to have confusion on what muscles to contract physically during practise of mulbandha. This arises from not understanding exact location of muldhara chakra.
If you touch your perineum an inch or so below the base of the penis and simulate ejaculation, you can exactly feel the activity of the ejaculatory sphincter muscles there. This is the exact location of muladhar chakra where one should focus/try to isolate while practising mulbandha or any other chakra practise.