The thoracolumbar fascia (lumboorsal fascia) is a deep investing membrane which covers the deep muscles of the back of the trunk. It is made up of three layers, anterior, middle, and posterior. The anterior layer is the thinnest and the posterior layer is the thickest. Two spaces are formed between these three layers of the fascia. Between the anterior and middle layer lies the quadratus lumborum muscle. The erector spinae muscle is enclosed between the middle and posterior layers.