Psoas 3

3rd February 2014

Koen Schoolmeesters continues to review the function of Psoas Major


Park R, Tsa H, Claus A, Cresswell AG, Hodges P,  2013

Changes in regional EMG-activity of the Psoas major and Quadratus Lumborum with voluntary trunk and hip tasks and different spinal curvatures in sitting,  JOSPT 43(2):74-82

Psoas Major - investigation on painfree subjects - EMG-activity of the Psoas Major with voluntary trunk and hip tasks (isometric MVC)

This paper showed that the EMG activity of Posterior Psoas was greater during trunk extension (prone) than during trunk flexion (crook lying) or hip flexion (supine). This suggests a role of Posterior Psoas at the lumbar spine; rather than involvement of Posterior Psoas during flexion of trunk or hip.

On the other hand EMG activity of Anterior Psoas was not significant different during trunk extension versus trunk flexion.  But Anterior Psoas EMG activity was significantly greater during hip flexion at 90° compared to trunk flexion while Posterior Psoas showed low EMG activity during hip flexion at 90°.  This leads to the suggestion that Anterior Psoas is more active as a hip flexor than active at the lumbar spine.   

We believe these findings should be interpreted and reproduced with some caution:

•    This study delivers evidence for a functional role of Posterior Psoas at the lumbar spine while the Anterior Psoas is more actively involved in movement control of the hip rather than the lumbar spine.  

•    The findings that Posterior Psoas is more affected by position of the lumbar spine than that of the hip, looks to be in contrast with biomechanical studies which conclude that Psoas Major can not generate torque at the lumbar spine (Bogduk 1992).  But maybe because of it’s poor biomechanical advantage there’s more necessity to activate to a higher proportion of MVC (%MVC)?

•    Based on the findings that Anterior Psoas is most active during hip flexion at  90° compared to Posterior Psoas, Internal Obliques, External Obliques and Erector Spinae, we cannot conclude that Anterior Psoas is the prime mover for hip flexion. Other hip flexor synergists  (e.g. m. Iliacus) weren’t indeed included in this study.

•    Furthermore in this study EMG activity of Posterior  and Anterior Psoas was recorderd during maximally voluntary contractions during 3 seconds. Because most of our functional activities are performed with submaximal recruitment, we cannot extrapolate these findings.
EMG activity of the Psoas major with different spinal curvatures in sitting

Investiation of EMG activity of Psoas Major’s anatomically different fascicles during 3 different sitting postures : slumped sitting posture, sitting with a flat spine and sitting in a short lordotic posture.  

In contrast to EMG activity of Posterior Psoas and Anterior Psoas, EMG of Quadratus Lumborum, Erector Spinae, External Obliques and Internal Obliques  did not differ between these 3 sitting postures.

In the slump posture all muscles were active to a similar low proportion of  MVC (<3%).  In the flat posture  Posterior Psoas, Anterior Psoas and Internal Obliques were active to a similar proportion of MVC (6%) and a greater percentage than the other muscles. EMG activity of Posterior Psoas was even greater in sitting in a short lordosis than sitting in a flat posture; Anterior Psoas activity was similar in both postures.

This data shows greater changes in activity of Psoas Major with subtle changes in spine curvature during sitting than those in other trunk muscles.  Both Posterior Psoas and Anterior Psoas were more active in flat sitting - and short lordosis postures compared to slump posture.  Only Posterior Psoas was even more active in the short lordosis compared to flat sitting postures. Posterior Psoas has a combined potential to extend/lordose the lumbar spine and maintain flexion of  the hip in the short lordosis posture. Based on this finding the authors suggest a dual function of Posterior Psoas at the spine and the hip.  Because we believe that the changes in hip flexion are indirectly a consequence of the increase of lumbar extension, we would minimize the influence of Posterior Psoas activity on the hip joint. Futhermore we believe that m. Iliacus  is the stabilizing hip flexor but this muscle was not subject of this research.