Someone asked yesterday about psoas strengthening and said “Isn’t it hard to do?"
It’s actually pretty easy. Do some iso holds lifting from 90 degrees.
Remember the psoas (and iliacus) are the only hip flexors that insert above the hip.
Anything starting with the leg fully extended (standing) will allow the use of the flexors that attach to the iliac crest (think TFL, rectus femoris, sartorious)