Jewels from Juker (1998). Insight into the Psoas Part One

Stu McGill was an author on this paper when it came out back in 1998.  At the time, I was one of Stu’s grad students putting electrodes onto anyone I could find for the price of Gyro sandwich.  I even burned (chemically and transiently) the thigh of a girlfriend at the time.  I knew how to treat the ladies.  Unfortunately, I never really picked Stu’s brain about this paper.  It was only relevant to me at the time because we were strongly questioning the necessity of double leg lifts as an exercise for the "lower abs".  We felt they were unnecessary to recruit the lower abs and too costly because of the compressive and anterior shear component applied to the lumbar spine. Our argument was that there is no difference between the upper section of the rectus abdominis and the lower section.  I still stand by it and the paper is here ( ). Regardless of my youthful oversight, I still love the paper and the ideas of sticking needles into the psoas.  It must feel awesome hence the “n” of only 5. Below are a few tidbits that will lead into future posts on psoas function. 1. the psoas was only minimally active (less than 5% of maximum) during a squat lift and while lifting 40 kg pails in both hands.  This suggests that during these tasks the psoas has a very negligible role for both stability demands and creating the primary movement.  This finding certainly questions the psoas importance as a stabilizer of the lumbar spine.  Although, part of Paul Hodges' research cabal recently put out a good paper (as usual) that showed that the psoas fired bilaterally (the only hip flexor muscle to do so) during an Active Straight Leg Raise (one leg only).  Again, suggesting its importance for stability.

2. the psoas was most active while standing with the hip flexed 90 degrees while a maximal force is applied against the knee in an attempt to extend the hip.  The psoas reached 100% max activity.  When the same action occurs while lying on ones back the average activity drops to approximately 57%.  This is cool although certainly not revolutionary to most.  It suggests that the psoas is not only working to flex the hip but to keep pelvis level – the psoas is working to create an ipsilateral lateral bending moment about the lumbar spine to help the contralateral hip abductors and probably the ipsilateral QL.

3.The limits of reciprocal innervation.  It is commonly stated that to inhibit one muscle we must activate its antagonist, aka. Sherrington’s law of reciprocal innervation.  In my opinion this is a pretty overused “law” in the Kinesiology field.  It is thrown around to justify treatments or explain dysfunction with really minimal if any research support.  The Juker paper took a look a the notion that the psoas could be inhibited during the sit up if the  hamstrings (a psoas antagonist) were activated. This hypothesis was not supported by Juker’s results, and in fact psoas activity was increased (28 % and 34 % MVC for the press heels style of situp) compared with that during bent knee sit-ups (17% and 28% MVC)

Big mouth comment:  Does the “law” of reciprocal innervation only apply when it is expedient.  We use it to justify PNF stretching, the “active” component of ART and to explain that a “tight or spasming” muscle is constantly inhibiting another muscle.  Yet, we ignore this law’s power during common activities.  Co-contraction of both agonists and antagonists is fundamental to stability and many movements (e.g. both the hamstrings and quadriceps fire simultaneously during running or squating – Lombard’s paradox. See link  I would really like to know what a real neurophysiologist says about this. I have always been  under the impression that reciprocal innervation/inhibition functioned at a reflexive level rather than at the voluntary activation level.

Part Two of this post will look at other “old” research that investigates Psoas function with questions about our clinical insights.

Super Jem of a Reference

Juker D, McGill S, Kropf P, Steffen T. Quantitative intramuscular myoelectric activity of lumbar portions of psoas and the abdominal wall during a wide variety of tasks. Med Sci Sports Exerc. 1998 Feb;30(2):301-10