The Psoas Major, the Lumbar Plexus, and the Dai Mai

Every December, I lead two back-to-back five-day cadaver labs for the University of Tampa’s Physician Assistant program.

In each lab, I lead half of the students through a full body dissection in five small teams, each working on a specimen. The following week, we repeat the process with the other half of the students. All in all, this involves a full dissection of 10 specimens and gives an amazing opportunity to see, reveal, study, and learn from the gift that the donors and their families have given.

Fig 1: The Psoas Major and the Lumbar Plexus. Netter’s Atlas of Human Anatomy, 7e

Although my job is to facilitate the learning of the students, each year there are key areas where I want to review and deepen my understanding—and, this year, those were the lumbar plexus, the nerves coming from this plexus, and their relationship to the psoas major.

Many practitioners are unaware that the psoas major has two layers—an anterior and a posterior layer—and that the lumbar plexus is housed between these layers in the proximal portion of the muscle. The anterior layer attaches to the vertebral bodies and discs of T12-L4, whereas the posterior layer attaches to the transverse processes of L1-L5.

Because the lumbar plexus is housed between these two layers of the psoas major, the nerves that exit the plexus then must penetrate the muscle as they travel through the deep posterior abdominal wall and then travel to their respective destinations.

Fig 2: Deadman, P., Al-Khafaji, M., & Baker, K. (1998). A manual of acupuncture (pp. 256-325). East Sussex, UK: Journal of Chinese Medicine Publications.

Some of these nerves, such as the ilioinguinal and iliohypogastric nerves, wrap around the waist (along with the subcostal from T12), pierce and innervate the inferior portion of the transverse abdominis muscle, and are (in my opinion) part of the overall anatomy of the dai mai.

Another nerve, the obturator nerve, pieces the psoas a bit more inferiorly, exits the pelvis at the obturator canal and then travels into the medial thigh. This nerve, if including the sensory portions, travels all the way to the posterior knee and I feel it follows the topography of the lower half of the kidney divergent channel.

It is worth noting that this secondary channel is said to intersect with the dai mai at L2, so the lumbar plexus anatomy is consistent with the channel description.

The important thing to understand is that an imbalanced psoas major can be involved with pain patterns affecting these nerves such as thoracolumbar junction syndrome or even obturator nerve entrapment and it is worth knowing how to assess this structure when dealing with these conditions, but really when dealing with many pain patterns of the low back, hip and groin.

Fig. 3: Anatomy of GB 27. Callison, M. Sports Medicine Acupuncture textbook. AcuSport Education. 2019

Let’s return to the neural anatomy and look at a very useful application.

For over two decades, Matt Callison has been using GB 27, a crossing point of the dai mai, to treat the psoas major. The intention of his needle technique is to stimulate one of the nerves of the lumbar plexus that passes through this region with the goal of reflexively stimulating the psoas major which, not only houses these nerves, but is also innervated itself by the lumbar plexus.

There are specific patient sensations that come from this technique that guide its success. Generally, this technique excels at stimulating an inhibited psoas, but it can also be used for an overactive psoas.

In Sports Medicine Acupuncture classes, however, we teach a more advanced, direct technique for the psoas (entering through the back) that is more appropriate in the majority of cases, but the details of this technique need to be carefully explained and demonstrated before someone attempts it.

This is just one application related to understanding this fascinating neuro-myo-fascial anatomy, and there are many more fascinating things about this muscle that we will be covering in an upcoming webinar—“The Psoas: Structure, Function and Treatment”—on January 15th.

In this class, we will look at the psoas major and its role in spinal stabilization and balance, its channel relationships, and its role in many lumbar and hip pain patterns. This 3-hour class will be a great ‘deep dive’ into this intriguing myofascial structure.