I’ve written about how reflex activity effected by the weight of the body returns from the legs to the lower torso, in my explanation of Yuanwu’s cryptic “turning to the left, turning to the right, following up behind” (1):
“Turning to the left, turning to the right”—stretch in the ilio-tibial bands sets off reciprocal innervation of the left and right sartorious muscles, and consequently reciprocal activity in the tensor and gluteous muscles. The result is a subtle “turning to the left, turning to the right” in an upright posture, and a stretch in the fascia behind the sacrum and the lower spine.
“Following up behind”—the combination of pressure from the “fluid ball” of the abdomen and stretch and resile in the fascia behind the sacrum and lower spine allows the vertebrae of the spine to find alignment, and permits the fascia behind the spine to provide support. (2)
My interpretation of “following up behind” is based on a study of the mechanics of the lower spine made by Gracovetsky, Farfan and Lamay (3). The authors speculated that in lifting weight, the abdominal muscles work against the extensors to align the vertebrae of the lower spine. They demonstrated through mathematical models that given an appropriate alignment of the spine, displacement of the lumbodorsal fascial sheet from its normal position by even a small fraction of an inch can provide critical support to the structure of the spine. Whether that displacement was to the rear, effected by hydraulic pressure created by the abdominals, or forward, as a consequence of action of the sacrospinalis muscles, the models were not sufficient to determine. The authors noted, however, that displacement to the rear by pressure created by the abdominals would at least in part explain the heightened activity of the abdominals in weight-lifting.
The study presupposed a flattening of the lumbar curve, like that of a person bent over to lift weight from the floor, but acknowledged that the control of the ligament system afforded by activity between the abdominals and extensors could not be directly accounted for in the models. My assumption is that a bent-knee posture like the lotus can engage the mechanism of ligamentous support the authors described, through alignment of the vertebrae of the spine.
The activity of the extensor muscles behind the sacrum might also bear on the displacement of fascia. Dr. H. F. Farfan wrote:
There is another peculiarity of the erector muscles of the spine. Below the level of the fifth lumbar vertebra, the muscle contracts in a compartment enclosed by bone anteriorly, laterally, and medially. Posteriorly, the compartment is closed by the lumbodorsal fascia. When contracted, the diameter of the muscle mass tends to increase. This change in shape of the muscle may exert a wedging effect between the sacrum and the lumbodorsal fascia, thereby increasing the tension in the fascia. This may be one of the few instances where a muscle can exert force by pushing. (4)
Farfan doesn’t address whether or not the “wedging effect” between the sacrum and the lumbodorsal fascia might contribute to the displacement of the lumbodorsal fascia behind the lower spine, nor does he discuss how the rotation of the tailbone and sacrum might affect the location of the tension produced by the “wedging effect” of the extensor muscles.
2) “Turning to the Left, Turning to the Right, Following Up Behind”, see table of contents
3) Gracovetsky, S., Farfan HF, Lamay C, 1997. A mathematical model of the lumbar spine using an
optimal system to control muscles and ligaments. Orthopedic Clinics of North America 8: 135-153
4) “Mechanical Disorders of the Low Back”, H. F. Farfan, p 183
End of “A Natural Mindfulness”