This is the second in a series of 11 blog post articles on psoas major function. See below for the other articles in this series.
The psoas major see bilaterally. Reproduced with kind permission from Muscolino, J. E., The Muscular System Manual: The Skeletal Muscles of the Human Body (4th ed.). (2017) Elsevier. Art work by Giovanni Rimasti.
The psoas major (see accompanying figure) is first and foremost, a muscle of the hip joint (5, 9, 12); therefore to determine its actions, we need to compare its line of pull at the hip joint in each of the three cardinal planes. Standard actions at the hip joint are considered to involve movement of the distal attachment, in other words, the thigh. These actions occur when the lower extremity is in what is known as “open-chain” position, with the distal segment, the foot, free to move. However, if the foot is planted on the ground and the lower extremity is in closed-chain position, the pelvis moves at the hip joint instead; when the proximal attachment moves instead of the distal attachment, this is called a reverse action (14). Therefore, a thorough examination of the psoas major at the hip joint involves consideration of its standard and reverse actions at that joint.
However, the psoas major is more complicated because it also crosses the lumbar spine, therefore we need to also examine its line of pull across the spine. As with the hip joint, the spine also allows motion in all three cardinal planes, so our examination of the psoas major must also consider the possible spinal actions in each of the three cardinal planes. What further complicates a clear understanding of the psoas major’s actions is the fact that the lumbar spine is not monolithic. There are many joints within the lumbar spine, each with its own axis of motion; therefore each of these joints must be considered separately. And finally, interposed between the spinal and femoral attachments of the psoas major is the pelvis. Therefore, the pull of the psoas major can affect the posture of the pelvis. Changing the posture of the pelvis can then change the posture of the lumbar vertebrae, which can change the line of pull of the psoas major relative the axes of motion of the lumbar spinal joints and therefore possibly change the action of the psoas major.
All of these factors help to explain why the psoas major can be so challenging to understand. In the following blog post articles in this series will be an examination of the functions of the psoas major at both the hip and spinal joints. In our discussion, we will consider some of the competing assertions for psoas major function by many of the leading authors in the field of kinesiology, and attempt to explain and perhaps resolve many of the reasons for the controversy regarding psoas major function.
Note: This is the second blog post article in a series of 11 articles on Psoas Major Function.
The 11 articles in the series are:
- Introduction & Muscle Biomechanics
- Biomechanics of the Psoas Major (Overview)
- Psoas Major Hip Joint Actions – Sagittal Plane
- Psoas Major Hip Joint Actions – Frontal Plane
- Psoas Major Hip Joint Actions – Transverse Plane
- Psoas Major Spinal Joint Actions – Frontal and Transverse Planes
- Psoas Major Spinal Joint Actions – Sagittal Plane
- Stabilization of the Spine by the Psoas Major
- Psoas Major and the Sacroiliac Joint
- Psoas Major and Fascial Pulls
- Summary of Psoas Major Function & Further Research
Note: This article is modified from an article originally published in the massage therapy journal (mtj): Psoas Major Function: A Biomechanical Examination of the Psoas Major. Spring 2013 issue.
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