Yes myth… the myth is that iliopsoas is functionally one entity. Iliacus and psoas and two individual, unique and independent muscles that share the same insertion on our thigh bone. Their important origins are totally unrelated. Akin to partners sharing the same household they can work together to achieve certain outcomes and at the same time have unique qualities that they express totally independent of the other. As both are strong ‘characters’ with important roles to fulfill they sometimes run into trouble and hence the infamous reputation!
Intentionally I will start with iliacus that it may have its moment in the limelight! Firstly, iliacus is innervated by completely different nerves to the psoas muscle. This is the same as saying they have different brains1 or at least minds of their own. Iliacus is innervated by the femoral nerve whose roots originate at L2, L3 (the second and third lumbar vertebrae). Iliacus originates on the anterior surface of our ilium (the iliac fossa) lateral to the centerline track of psoas and so plays no role in axial stability. It does cross over and become more superficial to psoas near their common insertion.
A very important functional difference is that iliacus is a single-joint muscle, i.e. it crosses only one joint. It has thereby one main purpose in life: to flex our hip joint. Its shape and location directly over the hip joint give it a mechanical advantage over every other muscle that really only contribute to this action. It’s the deep silent type, however, it does tend to suffer some symptoms of ‘short-muscle syndrome’. Iliacus is prone to facilitation in that it becomes over-active when trying too hard to make up for other muscles that do not fulfill their role. It works as a direct antagonist to gluteus maximum and synergistically with the other hip flexor muscles as well as those muscles in its kinetic chain and may attempt to carry the short fall if any of them become inhibited by trauma, stress or overuse.
An over-active iliacus will excessively draw the pelvis (one or both halves) forward. At its worst you may experience a pinching sensation in the groin, most often noticeable with deep hip flexion with adduction, e.g. when doing Marichyasana C.
Psoas is a very complex muscle that deserves much more detail than is possible in this blog, however, let’s look at some relevant aspects to this particular topic.
Psoas major is a multi-joint muscle and crosses and therefore acts upon eight different joints. This count includes six vertebrae (not counting the facet and vertebral disc joints as separate joints) plus the sacroiliac joint and the hip joint.
The superficial and deep layers together not only span the anterior surface the last thoracic to the last lumbar vertebrae (T12-L5) but also attach to most of the lumbar discs. This gives us some indication of its important role in stabilisation of the lumbar spine anteriorly, i.e. in flexion and rotation. Psoas is an important postural muscle that additionally stabilises the pelvis and significantly influences sacroiliac joint motion. Its intimate connection to the thoracic diaphragm muscle via the medial arcuate ligament means that with every breath our diaphragm and psoas work together to provide spinal stability… And then additionally psoas contributes to the same actions as iliacus.
This ability to multitask makes the psoas susceptible to inhibition – a bit like working eight jobs at the same time and becoming stressed, overwhelmed and/or exhausted! Psoas’ high proportion of slow-twitch fibres enable it to work for long periods of time at low-force, like the feminine yin energy. Because of its central role in spinal stability an inhibited psoas will cause our motor control system to compensate in a multitude of possible ways as even our limb movements depend on spinal stabilisation. Interestingly a common over-compensator for the psoas muscle is her synergistic partner, iliacus! As spinal stability is one of the highest priorities of our nervous system, when the psoas becomes inhibited, her partner in hip flexion (she has others J for her other tasks), iliacus attempts to compensate and often ends up facilitated. Other muscles that may attempt to compensate for psoas inhibition is the psoas on the opposite side of the body and the neighbouring quadratus on the back.
If your psoas is tight, it is an indication of a lack of spinal stability in flexion rather than the psoas itself being the problem. Should you proceed to stretch or release a tight psoas, you may get momentary relief but you will not repair its ability to stabilise your spine. The common reaction for a weak muscle after recoiling from a stretch is to tighten even more! This is the intelligence of your motor control system kicking in to protect your spine and the spinal cord it encases.
Psoas is innervated by direct branches of the spinal nerves L1-L42. As we develop these nerves become deeply embedded within the belly of the muscle. Its embrace of the lumbar nerves, its attachment to the intervertebral discs and its important role in stabilisation mean the psoas is often involved in cases of low back and sacroiliac joint pain.
What does psoas major and iliacus have in common? Their tendons blend and they share the same insertion onto the lesser trochanter of the thighbone (femur). Psoas contributes synergistically to iliacus’ role of hip flexion. They also share the same fascia along with the kidney. This close association to the kidney links them to the kidney’s emotions of courage and fear. When we engage these muscles together they contribute to the movement pattern that curls us up into the safety of the foetal position. The healthy condition of our iliacus and psoas muscles enable us to balance the hip joints and pelvis and to lengthen the spine, the same posture we adopt when we feel courageous.
One of the aspects that I love about studying and teaching yoga anatomy is the relatability of those relationships within the myofascial system to our human actions and interactions. When we work together an extraordinary synergy is created and yet each of us is unique in our expression and what we have to offer. Our contribution as an individual is as vital to the whole as our role within the whole is integral to its balance, health and vitality.
 This terminology is borrowed from Paul Chek.
 Different sources will cite slight variations in levels between L1-L4.
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