The psoas major and rectus femoris are two of the body main hip flexors (sartorius and tensor fasciae latae are others).
The Psoas Major
The psoas major is the most important muscle in the body. It walks us forward through life, it warehouses all of our unprocessed emotions and it is goes a long way to holding the spine upright on top of the pelvis. The psoas at the front and the gluteus maximus and piriformis in the back are connecting the legs to the pelvis, and between them, holding the spine aloft if the psoas is well aligned and functioning as designed.
The Rectus Femoris
The rectus femoris is one of the four quadriceps muscles. The four quadriceps muscles all meet to insert into the tibia of the shin. But the rectus femoris is the only one of these muscles that connects to the pelvis. The others—vastus lateralis, medialis and intermedius—all connect to the femur or thigh bone. The quadriceps is essentially an extensor of the knee, and only the rectus femoris plays a role in hip flexion.
Anyone who reads the blog knows that I am obsessed with getting people to stop tucking the pelvis. The other piece to untucking is getting the thighs to move directly under the hips. This thigh alignment is key to getting the psoas to work as it is designed and the for the rectus femoris to work successfully as a hip flexor.
The psoas acts like a pulley system in the body. A pulley is a simple machine and viewed from the side, the hip bone is the pulley and the psoas is the rope. The pulley work of the psoas facilitates the reciprocal inhibition that allows the extensor muscles of the spine to lengthen up.
The psoas can only act like a pulley if the legs are under the hips. If the legs are where they should be, the psoas moves down from its origin at the base of the rib cage/top of the lumbar spine, to move forward crossing the rim of the pelvis, and then moves back again to insert on the lesser trochanter a knob of bone on the back half of the thigh.
It is the back/ front/back arrangement that foster the psoas ability to help support the spine on top of the pelvis. For this to happen the thighs have to be under the hips. When the pelvis tucks under and the thighs move forward taking the psoas with them, the tension across the rim of the pelvis is lost and much of the psoas support capabilities go with it.
When the thighs go forward a great deal of stress and pressure is put on the rectus femoris muscle basically pushing it forward. When this happens the rectus femoris is called upon to support the upper body because the power of the psoas is taken out of the picture.
The Rectus Femoris Tendon
The rectus femoris tendon is really what bears the brunt of this powerful misalignment and rears its ugly head for many people when they try to do core work when lying on the back. This tendon when it is over stretched and overburdened from the weight of the upper body sinking into it tends to scream out in pain.
This happens literally as the tendon can pop like a steel cable and pretty much shut down all attempts to either engage the quadriceps or work in concert with the psoas. This happens most often in poses like navasana or one of my key exercises feet three inches off the floor.
This has all been yet another cry from this blog to stop tucking your pelvis and start getting your thighs to move under your hips. The effort will pay off in a big way.