I’ve been studying a couple of illustrations Apech turned me onto–from the temples in Egypt, a long time ago. You might find this useful, with regard to the sacrum and L5:
I would suggest that the reed that the god is holding on either side of the pillar represents the horizontal ilio-lumbar ligaments, that engage as the spine is relaxed upward from the sacrum in the movement of exhalation. The presence of the god is an indication that the tensile support of the ligament is realized not through the direction of any conscious activity, but solely through the experience of the location of awareness and the ability to feel in the necessity of breath.
The hieroglyphs on the top of the pillar are in praise of the king of Egypt, but they also depict an orb like the sun, which is perhaps the Egyptian “akh” or consciousness freed of any fixture to location.
The toes of the god rest against either side of something shaped like the sacrum of the body; close-ups show that not only is the footrest of the god the shape of the sacrum, but it is also segmented in five parts like the sacrum.
The nerves which exit between the fourth and fifth lumbar vertebrae and the first vertebrae of the sacrum allow an ability to feel in the lower legs and along the soles of the feet, right to the surface of the skin. As feeling for the placement and orientation of the legs and feet is absorbed into the sense of location in the occurrence of awareness, the alignment of vertebrae may allow feeling for the placement and orientation of the sacrum to enter awareness, and likewise for the bottom-most part of the spine.
That the body can be held upright in zazen solely through the interplay between a sense of location, an ability to feel, and relaxation in the movement of breath is illustrated in another graphic from the walls of the temples of Egypt:
The goddesses Isis and Nephthys kneel on what may be a woven material, one knee of each goddess resting against what might be construed to be a representation of the first lumbar transverse processes. This is exactly the vertebrae supported by the horizontal ilio-lumbar ligaments, and here a connection between the placement and orientation of the legs at the knees and support for this vertebrae is suggested. The hands of the goddesses frame the lower spine, while the headdresses, the baboons on either side of the image, the hands of the baboons and the hands of the “ankh” (the cross with an oval) all emphasize an upward extension. There are two mounds outside of the benches on which the goddesses kneel, very much in the shape of the pelvic wings, while the dunes the baboons walk on form a space whose interior resembles the cavity of the chest. The large dark border along the top of the image strongly resembles the diaphragm. The “ankh” or looped cross is speculated by some to be a representation of the vertebrae of a cow, and in my mind it’s entirely possible that this shape is used to represent the cranial-sacral fluid system as described in modern cranial-sacral osteopathy; regardless, the occurrence of consciousness freed of fixture to location is again depicted in the orb of the sun, here framed by hands extended upward from the ankh, and touching on the central cone of the diaphragm-like dark line.
The benches the goddesses kneel on I believe represent the ligaments that connect the sacrum to the pelvis, and the central point of the illustration would be the upward support provided as feeling opens for the orientation of the legs at the knees and the weight that rests on the ilio-sacral ligaments in the relaxed movement of breath.
The short rendition would be that feeling in the soles of the feet corresponds with feeling at the ilio-sacral joints, especially as a sense of roll is realized at the location of awareness and feeling throughout the body informs the sense of location. Roll has an aspect that is stretch and activity in the obturators, the muscles that hammock the hips away from the pelvis. Feeling for placement and motion in the knees corresponds with feeling at L5-L4-L3; I have an excellent dermatone chart from “Lower Back Pain” by Calliet that shows feeling on the surface of the skin on either side of the calf corresponds with L5-L4, and feeling above the knee with L3. This ability to feel is especially connected with yaw at the location of awareness, and the action of sartorius to swing the wings of the pelvis and engage gluteous and piriformis under the sitbones.
Even shorter rendition: the information coming from muscles, tendons, and joints informs the ability to feel necessary to relax the movement of breath, and contributes to the induction of a hypnogogic state with its accompanying singularity in the sense of location.