Note: This is the eighth blog post article in a series of 11 articles on Psoas Major Function. See below for the other articles in this series on psoas major function.
Thus far, by examining the lines of pull of the psoas major, it is clear that it can create flexion and extension of the lumbar spine. But because the psoas major is so close to the spine, its moment arms for these motions are small compared to other musculature such as the rectus abdominis anteriorly for flexion, or the erector spinae posteriorly for extension. Regarding the argument over whether the psoas major’s role is spinal flexion or extension, Hamilton wisely states: “…it seems likely that the differences are not of great importance. Frequently, when there is lack of agreement regarding movement, one may safely assume that the true function of the muscular contraction, with reference to the joints in question, is more likely to be stabilization or balance than purposeful movement.” (5).
This view that the psoas major acts primarily to stabilize the spine is shared by many others (2, 12, 19, 21, 25, 26, 29). Neumann states that the psoas major is “…neither a dominant flexor nor extensor of the lumbar region, but rather a dominant vertical stabilizer of the region.” (19). Brunnstrom’s states that muscles close to the spine act like guy ropes supporting an upright pole. “When the pole starts to tip, the extension of the ropes on the opposite side increases.” (28).
The effectiveness of the psoas major as a stabilizer can be validated by noticing that much of its mass crosses either extremely close to or directly over the axes of motion (see Figure 12). Therefore, contraction of the psoas major would create an axial compression that would act to stiffen and stabilize the lumbar spine (see accompanying figure). Some sources assert that the importance of the psoas major as a stabilizer is specifically linked to hip flexion (2, 12). Also, the fact that the psoas major is unipennate in design (as mentioned previously) further supports its role as a stabilizer (2, 31).
Stabilization and Compression of the Spine
Healthy functioning of a joint demands that the joint is mobile and stable. Therefore, motion is not the only importance of a muscle. However, this fact can be easily overlooked because muscle function is usually stated by listing the concentric movement actions of the muscle, not the isometric stabilization contraction functions. Recently, though, with the advent of Pilates and core strengthening in general, the awareness and appreciation of spinal stability has increased. In this role, the psoas major likely plays an important role.
Some sources are concerned though about the effect of the spine of psoas major compression/stabilization. Compressing the lumbar spine means compression of the disc joints, with the physical stress that is associated. McGill warns: “Caution is advised when training this muscle due to the substantial spine compression penalty that is imposed on the spine when the psoas is activated.” (12). Similarly, Sahrmann cautions: “Clinical implications…are to minimize iliopsoas activity in the exercise program when compression and anterior shear are the sources of the patient’s pain.” (25). And Oatis believes that low back pain that occurs with hip flexion is probably due to psoas major contracting and causing compression of the lumbar spine (20). The negative effect of psoas major compression upon the spine can also be likely inferred from the fact that disc herniation and low back pain have been shown to cause “…significant reduction in the psoas major activity bilaterally…” (26). This is likely the body’s attempt to reduce the physical compression stress of psoas major contraction that is likely contributing to the pain and dysfunction.
However, there is not universal agreement that the effects of the psoas major stabilization are necessarily deleterious. Gibbons states: “A mechanism to simultaneously flex the hip and stabilize the lumbopelvic region is needed. It does not seem logical that a muscle such as PM (psoas major) would have a detrimental effect to the lumbopelvic region.” (2). It is likely that the cost/benefit ratio of psoas major compression stabilization would vary from individual to individual and in the end would be a clinical decision based on your client’s specific presentation. For those clients with pathologic disc, advanced degenerative joint disease, or other spinal conditions, caution should be exercised when recommending any activities that would greatly increase psoas major engagement.
Note: This is the eighth 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.