Here are prerequisites for the context of my mini-article:
1) As it is represented here by D. GORMAN's drawing, The Quadratus Lumborum (QL) is a complex multi-layered design with criss-crossing fibers groups that connect torso to pelvis, lumbar spine to pelvis and occasionally lumbar spine to last floating rib.
2) In traditional anatomy, the QL is described as a structure that creates lateral flexion of the lumbar column towards the torso (and vice and versa) when contracted unilaterally. This refers to the function of the QL when it contracts: A/ concentrically (gets shorter) in reaction to the electrical stimulus of the anatomist B/ with the torso and the pelvis aligned on the same frontal plane (meaning when you observe its effect on a body lying on a horizontal plane)
It seems that the ECCENTRIC and tensile ACTIVITY of the QL is a physiological phenomenon that is underestimated in medical circles because of the way muscle functions have historically been tested: on dead and lying bodies.
In other words, in our daily life, when we walk, the essential roles of the QL would be to manage and decelerate THE SIDE-TO-SIDE DROP of the pelvis, as well as THE TWIST (counter rotational motion around vertical axis) between torso and pelvis-lumbar column that, for most people, happens when we take a step.
This decelerating activity, in addition to reducing stresses in the joints, prepares the torso to oscillate back in the opposite direction (on time for the next step) and prevent the pelvis from dropping too low (so that the foot can clear the ground) when the leg is swinging from the back to the front.
Another important benefit of the eccentric activity would be to store energy in the tissue in order to save significant energetic costs.
The TWIST between torso and pelvis and the side to side drop of the pelvis shift temporarily the structural relationship between pelvis-lumbar column and ribcage: the last floating rib vire away from the iliac crest which reveals a tensile activity of the QL that is invisible on a static body.
In that light, the QL is then seen as a discret and wonderful structure that nurture the transversal oscillations and side to side compensation of the ribcage and pelvis, supportive of the thoraco-lumbar fascia, erector spinae, obliques and transverse abdominals, the skin etc...
