the multifidi and the Transverse Abdominis have a close relationship. I found a link to a great textbook. The relevant pages start at 43 https://books.google.com/books?id=X70TfQt-0kcC&printsec=frontcover#v=one….
The multifidi is a group of muscles that are about as short and as close to the spine as muscles get. Anyone can potentially do an exercise that involves the these muscles. But the real idea behind training them is to teach clients how to activate them intentionally. (Just like all of the “core” muscles.) If you really learn to connect the brain to these muscles, you might even be able to selectively activate them. It would take me quite a bit of time to explain it all to you here. I am sure that there are search-able websites on line that you can find some information on this.
I teach a CEC course on the core and activation of the core, etc. I only teach it in person, as this is very intricate stuff and it is easy to feel like you are doing it correctly when you are not. If you are ever planning a vacation to the Big Island of Hawaii, get in touch. My new website is almost finished, it is www.hawaiifitnessacademy.com .