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November 5, 2009

I have heard many times in casual conversation and case demonstrations at


Picture courtesy of Primal Pictures

 various soft tissue seminars when observing/discussing running gait, “if the hip externally rotates during the swing phase that means that the athlete has an overactive psoas.”  The idea here (better described as a myth) is that because the psoas muscle makes its distal insertion into lesser trochanter of the femur, it acts to externally rotate the hip as well as to flex it.  Thus at the end of stance phase of the trailing leg, during the first half of the swing, and over active psoas will cause external rotation of the hip in addition to its action of producing hip flexion.

WAY back in 1999, Skyrme et. al, in the journal ‘Clinical Anatomy’ (12:264-264, 1999) demonstrated that this is in fact not the case:

“The psoas and iliacus muscles, along with their common insertion, were isolated by dissection in six adult cadaveric specimens. The action of psoas muscle was assessed by pulling the muscle along its long axis and then observing the effects on rotation of the femur, with a visual estimation of the rotation in degrees.     Conclusion:  Traction applied along the long axis of the muscle produced hip flexion with NO rotational component.”

To further debunk some common myths Yoshio, et al., in 2002 (Orthop Sci, 2002;7:199-207) concluded that the primary role of the psoas major was for lumbar stability and that the psoas major contributed VERY LITTLE to hip flexion. He explained that the primary role for the psoas major is at the hip for stability. This was achieved through maintaining the femoral head in the acetabulum. 

Well then psoas must act to flex the lumbar spine….right?  Based on his studies Bogduk (Clin Biomech, 1992;7:109-19) does not believe the attachment of the psoas muscle has a long enough lever to act as a prime flexor of the lumbar spine.  His studies indicate that in erect posture, the psoas exerts an extensor moment on the upper lumbar spine and a flexor moment on the lower segments.  The major forces acting on the lumbar spine are compression and anterior shear forces.  The psoas has a primary stability role at the lumbar spine for axial compression and it has a minimal movement function on the lumbar spine.

To quote my recent article in Canadian Chiropractor “Medical research can only be considered effective if it changes the practices and/or mentality of its intended audience in a way that, ultimately, benefits patient care.”

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