Ok! Now the psoas. This is a tricky muscle. The whole inner sling can be tricky because it is connected to our breath. By inner sling, I am referring to the "Deep Front Line" discovered by Thomas Meyers in his development of what he calls the "Anatomy Trains". All muscles are invested in fascia, but the trains define a continuous pathway in the body that maintains a more direct relationship. We must keep in mind that everything is connected despite our determination to divide. We must also keep in mind that we can and do make our own beautiful and complicated connections despite the neatly packaged pictures we see in text books. In a workshop I attended with Gil Hedley in 2012, he said there is only one muscle. The scalpel divides. Regardless, we use divisions to help us to understand various relationships, communications, and coordinations in our body. Hopefully, it is more commonly understood today that all systems make up the whole and are intricately intertwined.
The psoas originates from the 5th lumbar to often the 12th Thoracic vertebra on the bodies and transverse processes. It inserts onto the lesser trochanter of the femur. I am of the camp that believes that the psoas should assist as a stabilizer of the lumbar and is important in holding the femoral heads in the acetabulum. Gibbons and Comeford 2001 has cited the posterior fascicles of the psoas as local stabilizers and the anterior fascilcles as global stabilizers. Some researchers believe that the role of the psoas is not flexion but purely stability. I am not sure if the argument has been settled yet. I have a hard time believing the psoas is not flexing. Regardless, in my practice, I can surely feel that connecting to my weaker side psoas before moving global muscles gives me a lot more stability and ease of motion. On the stronger psoas side it just happens. I don't have to think about it.
If the tone and strength and communication between the multifidi, transverse abdominus, and the psoas is optimum, the abdominals don't have to work so hard to hold you up. The multifidi originate on the transverse processes of the spine as well as the posterior surface of the sacrum, and insert into the spinous processes of the spine 2 to 4 vertebrae superior to the origin, including all spinous processes. If you imagine elastic bands on the transverse processes pulled back and up to the spinous process above, you get the idea of multifi. If this is not working properly, either the abdominals work too hard to gain stability, or the upper fibres of the psoas work too hard to gain stability, or the back starts to sway under the force of the psoas, or the psoas stops working and the other muscles take over. There are other possibilities and usually there are combinations of things happening. That is why in my work, you have to be a good detective.
I am going to go back to the idea of the abdominals "working too hard" as this may be a weird thought for many. Sometimes we need to strengthen abdominals and that can really help, but that is not the whole picture when we talk about core support. If the abdominals get to rigid to compensate for weaker core muscles, this will challenge our ability to have a full and natural breath and put unnecessary pressure on our organs. Pressure on our organs, challenges our internal processing.
Now I am going back to the psoas - multifidi relationship. Use your transverse processes like handle bars from anterior to posterior (back and up to the spinous processes above) to hold the spine steady as you lift your leg up to shorten the psoas and as you extend your leg back to lengthen the psoas. See what you notice. The movement at a joint is a relationship between the movers and the stabilizers. So with that in mind, it is not the leg moving that is important but that the relationship between the mover and stabilizers is coordinating towards the same goal.
Play and have fun.