In part I of this series, Chris Mallac looks at the detailed anatomy and function of psoas major, and the role it plays in lumbo-pelvic stability.
The anatomy and function of the psoas major (PM) has received considerable attention in the literature over the last decade. More recently however, the traditional model and understanding of the PM - and its interaction with the iliacus as being a hip muscle that contributes to hip flexion force - has been questioned. Newer models based on cadaveric and biomechanical studies suggest that the PM relationship with the iliacus is not as intimate as first thought. Its role as a dominant hip flexor may be relatively unimportant, while the notion of its primary role as a lumbar spine stabiliser has gained popularity(1).
The purpose of this two-part article therefore is to dissect the literature on the PM both in terms of its anatomy and its function, and outline the role it plays in lumbar spine stability and presenting pain and pathology in and around the lumbopelvic area.
Proximal anatomy
The PM is a long unipennate muscle that has one fascial attachment (via the diaphragm) and two fibrous attachment onto the spine (see figure 1)(2). In terms of its upper fascial attachment onto the lumbar spine and the diaphragm, the PM attaches to the diaphragm via the medial arcuate ligament. This is a tendinous arch in the fascia that covers the PM, with its fibres converging to a tendon that ascends to the diaphragm(2).
The diaphragm itself attaches to the spine via the right and left crus. They attach to the anterolateral component of the upper three lumbar vertebral bodies. The crus and their fascia overlap the PM and appear to be continuous with this muscle until they come more anterior and blend with the anterior longitudinal ligament of the spine(3). This PM-to-diaphragm attachment may have implications for lumbar spine stability as discussed later in the text.
The more traditional understanding of the proximal fibrous attachments is that the attachments of the PM are arranged as fascicles. They attach anteriorly to the L1 - L5 transverse processes (these are the posterior fascicles) and anteriorly from the T12 – L1 disc to the L4 - L5 disc (these are the anterior fascicles). The L5/S1 disc has no attachment(4).
The fiber length ranges between 3-5 cm within the posterior fascicles, and 3-8 cm in the anterior fascicles. The fascicles rotate medially so the posterior fascicles become anterior and the anterior become posterior as they descend down and laterally towards the pelvic rim(2,5). These short fascicle lengths mean that the fibre length does not extend the full length of the muscle.
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