Differential activity of regions of the psoas major and quadratus lumborum during submaximal isometric trunk efforts.

Controversy exists regarding the function of psoas major (PM) and quadratus lumborum (QL) at the lumbar spine. The functions of discrete regions of PM and QL were studied during trunk loading tasks. Twelve healthy participants performed isometric trunk loading tasks in various directions in upright sitting. Fine-wire electromyography (EMG) electrodes were inserted under ultrasound guidance into PM fascicles arising from the transverse process (PM-t) and vertebral body (PM-v) and the anterior (QL-a) and posterior (QL-p) layers of QL on the right side. Although right PM-t and PM-v were both active during right lateral-flexion trunk efforts, their activity was opposite in the sagittal plane, with greater PM-t towards extension and PM-v towards flexion. QL-a and QL-p were similarly active, though QL-p was active to a greater percentage of MVC during right trunk lateral-flexion efforts. Activity of QL-p was modulated with respiratory phase during the loading tasks with trunk efforts towards the right lateral-flexion/flexion and right lateral-flexion directions. These findings provide novel understanding of the unique activation of discrete regions of PM and QL. These differences must be considered in future EMG studies to better understand the function of these deeply situated trunk muscles in the control of the lumbar spine.