Well, multifidus is often spoken about as though it were only one muscle, but it's actually many, many individual muscles (collectively
called multifidi) running the length of the spine from the low back to the neck.
This means we find that happy medium between the two extremes of our pelvic motion where we have a gentle lumbar curve that gives a slight lift to our tailbone allowing our
multifidi muscles to engage, allowing our transversus abdominus muscle to pull against solid interlocked vertebrae, and allowing our pelvic floor muscles to pull against a solid tail bone to contract when needed and come back to a neutral resting position when not being
called upon.