Little is known about the contribution of the deeply situated psoas major and quadratus lumborum muscles to control of the lumbar spine and pelvis, despite the potential of these muscles to provide a substantial contribution given their large cross-sectional areas and segmental attachments to the lumbar vertebrae. Adding complexity to the interpretation of the function of these muscles is their complex anatomy with each muscle composed of multiple anatomically distinct regions with unique muscle attachments and moment arms. This complexity is likely to underpin the contrasting views of the muscle’s function and a possible association with low back pain. This latter issue has led to contradictory assumptions in the clinical management of the muscles. A fundamental issue that requires resolution is understanding of the function of the multiple fascicles of the psoas major and quadratus lumborum muscles in healthy individuals and the potential for changes in action of the discrete regions of these muscles in low back pain with particular attention to the issue of redundancy of trunk muscles (multiple strategies to achieve a similar goal) and the heterogeneous nature of the low back pain population. The overall objectives of this thesis were to investigate activity of anatomically discrete regions of psoas major and quadratus lumborum muscles, and to examine whether this differs between individuals with and without low back pain. Techniques were developed to make unilateral electromyographic recordings of myoelectric activity of discrete regions of the psoas major and quadratus lumborum muscles using intra-muscular fine-wire electrodes, inserted with ultrasound guidance. The major fascicle bundles selected for evaluation were those of psoas major arising from the transverse process and the vertebral body, and those of the anterior and posterior layers of quadratus lumborum on the right side of the back. Several experimental paradigms involving postural and functional tasks were used to study different aspects of the potential differential function of these anatomical regions. In separate experiments the results showed that discrete regions of these muscles are activated differentially and in a manner that is explained by subtle differences in moment arms. The studies showed that discrete regions exhibited; (i) unique relationships to the direction of force when an isometric force was generated in multiple directions; (ii) differences in activation between postures with different spinal curvature in sitting; (iii) different activation in maximal hip and trunk efforts; (iv) different activation in advance of a predictable perturbation to the trunk from movement of an arm; and (v) subtle differences in association with breathing. With respect to low back pain, the data revealed a redistribution of activity within (including regions of psoas major and quadratus lumborum) and/or between trunk muscles, particularly with respect to those with an extensor moments in individuals with low back pain. Modified regional activity of psoas major and quadratus lumborum was present in some, but not all, individuals with low back pain. These changes included a greater bias in activation of the specific regions of psoas major and quadratus lumborum towards trunk extension and a less bias towards extension for regions of psoas major in low back pain groups with low and high erector spinae activity, respectively. Together, these studies provide new understanding in the unique control of discrete regions of psoas major and quadratus lumborum and differential changes in the regions of these muscles in some individuals with low back pain. These data provide a foundation to consider evaluation of the potential benefits of rehabilitation of activation of psoas major and quadratus lumborum, but with the necessity to consider the individual and regional differences in muscle activation.