Upside Down and the Outside goes In

Inverted on Sleg's inversion bar for a few minutes this morning.  I recalled something the massage therapist was telling me about muscle cells and how, scattered through every muscle are spindle cells whose function is to seize up if the muscle is over-stretched, to keep it from tearing.  To get into the deeper stretching - at which level it really feels like releasing the years of never-stretched tightness, you have to slowly settle to bypass the spindle cell reflex tightening.  I'm sure i'm off on the details but this is the picture I have in my mind and am navigating into the deeper channels using it as my map.

So me inverted, Mindfolded and finding the focus of tightness to be in my hips and groin and as I breathed into these places, i could slowly feel my skeleton separating, opening and closing with my breath.  Even as I sit on the floor typing this i can feel the pelvic bones tentatively settling into a slightly more open position.  Now the next thought is how to continue to engage these muscles which have been held tight in compensatory positions for years without over-doing it and injuring myself...how does one strengthen the psoas?