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The superficial and deep lamina of the posterior layer of the thoracolumbar fascia have been studied anatomically and biomechanically. In embalmed human specimens, the posterior layer has been loaded by simulating the action of various muscles. The effect has been studied using raster photography.To study the role of the posterior layer of the thoracolumbar fascia in load transfer between spine, pelvis, legs, and arms.It has been determined whether muscles such as the gluteus maximus, latissimus dorsi, erector muscle, and biceps femoris are functionally coupled via the thoracolumbar fascia. The caudal relations of the posterior layer of the thoracolumbar fascia have not been previously studied.Dissection was directed to the bilaminar posterior layer of the thoracolumbar fascia of 10 human specimens. The superficial and deep lamina were studied using visual inspection and raster photography. Tension to the posterior layer of the fascia was simulated by traction to various muscles and measured by studying the displacement in the posterior layer.Traction to a variety of muscles caused displacement of the posterior layer. This implies that in vivo, the superficial lamina will be tensed by contraction of various muscles, such as the latissimus dorsi, gluteus maximus and erector muscle, and the deep lamina by contraction of the biceps femoris. Caudal to the level of L4 (in some specimens, L2-L3), tension in the posterior layer was transmitted to the contralateral side.Anatomic structures normally described as hip, pelvic, and leg muscles interact with socalled arm and spinal muscles via the thoracolumbar fascia. This allows for effective load transfer between spine, pelvis, legs, and arms-an integrated system. Specific electromyographic studies should reveal whether the gluteus maximus muscle and contralateral latissimus dorsi muscle are functionally coupled, especially during rotation of the trunk. In that case, the combined action of these muscles assists in rotating the trunk, while simultaneously stabilizing the lower lumbar spine and sacroiliac joints.