This article focuses on the (functional) anatomy and biomechanics of the pelvic girdle and specifically the sacroiliac joints (SIJs). The SIJs are essential for effective load transfer between the spine and legs. The sacrum, pelvis and spine, and the connections to the arms, legs and head, are functionally interrelated through muscular, fascial and ligamentous interconnections. A historical overview is presented on pelvic and especially SIJ research, followed by a general functional anatomical overview of the pelvis. In specific sections, the development and maturation of the SIJ is discussed, and a description of the bony anatomy and sexual morphism of the pelvis and SIJ is debated. The literature on the SIJ ligaments and innervation is discussed, followed by a section on the pathology of the SIJ. Pelvic movement studies are investigated and biomechanical models for SIJ stability analyzed, including examples of insufficient versus excessive sacroiliac force closure.

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Meaningful classifications exist for sagittal plane posture in young adolescent boys, both on gross body segment and lumbopelvic level. In terms of clinical importance, that is, low back pain and neck pain prevalence, postural subgrouping strategies based on the orientation of gross body segments are suggested to be superior when compared with lumbopelvic grading.

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Movement and stability of the lumbosacral region is contingent on the balance of forces distributed through the myofascial planes associated with the thoracolumbar fascia (TLF). This structure is located at the common intersection of several extremity muscles (e.g. latissimus dorsi and gluteus maximus), as well as hypaxial (e.g. ventral trunk muscles) and epaxial (paraspinal) muscles. The mechanical properties of the fascial constituents establish the parameters guiding the dynamic interaction of muscle groups that stabilize the lumbosacral spine. Understanding the construction of this complex myofascial junction is fundamental to biomechanical analysis and implementation of effective rehabilitation in individuals with low back and pelvic girdle pain. Therefore, the main objectives of this study were to describe the anatomy of the lateral margin of the TLF, and specifically the interface between the fascial sheath surrounding the paraspinal muscles and the aponeurosis of the transversus abdominis (TA) and internal oblique (IO) muscles. The lateral margin of the TLF was exposed via serial reduction dissections from anterior and posterior approaches. Axial sections (cadaveric and magnetic resonance imaging) were examined to characterize the region between the TA and IO aponeurosis and the paraspinal muscles. It is confirmed that the paraspinal muscles are enveloped by a continuous paraspinal retinacular sheath (PRS), formed by the deep lamina of the posterior layer of the TLF. The PRS extends from the spinous process to transverse process, and is distinct from both the superficial lamina of the posterior layer and middle layer of the TLF. As the aponeurosis approaches the lateral border of the PRS, it appears to separate into two distinct laminae, which join the anterior and posterior walls of the PRS. This configuration creates a previously undescribed fat-filled lumbar interfascial triangle situated along the lateral border of the paraspinal muscles from the 12th rib to the iliac crest. This triangle results in the unification of different fascial sheaths along the lateral border of the TLF, creating a ridged-union of dense connective tissue that has been termed the lateral raphe (Spine, 9,1984, 163). This triangle may function in the distribution of laterally mediated tension to balance different viscoelastic moduli, along either the middle or posterior layers of the TLF.

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Psychosomatic symptoms and pain comorbidities had the strongest association with 1-month period prevalence of spinal pain in young adolescents, followed by factors from the physical and psychosocial domains. The role that "physical factors" play in non-adult spinal pain may have been underestimated by previous studies.