between the axial skeleton and the lower extremities therefore gives the posture the shape of a right tetrahedron. The triangle formed by the lower extremities provides a firm base for the posture, the right angle between the base triangle and the upright axis of the spine is what makes the posture upright, while the upper extremities stabilize the upright axis and keep the torso from pitching forward.
the axial muscles
The erector spinae, the muscle complex that attaches between various vertebrae on the posterior side of the body and to the pelvis (figs. 4.14, 5.5, and 8.14), holds the back axis of the tetrahedron in a straight line, flattening the thoracic kyphosis and providing lift to that part of the vertebral column. Since part of its inferior attachment is to the pelvis, the erector spinae also acts to increase the lumbar lordosis. In the classic cross-legged sitting posture, it is the loremost muscle group that counteracts slumping (that is, the flattening of the lumbar lordosis followed by its further rounding to the rear). The erector spinae also acts to create what we call an axial extension, which is an overall lifting of the axis of the body.
Two more muscles—the quadratus lumborums—-lie anterior to (beneath, when approached in a dissection) the lowermost portion of the erector spinae. These muscles, one on each side, take origin from the posterior part of the iliac crest, and the sacrum, and insert on the upper lumbar vertebrae and the 12th rib (figs. 2.7, 3.7, 5.5 and 8.14). They function as synergists to augment the function of the erector spinae, acting roughly like a string on a bow. When they contract, they bend the bow of the lumbar curve forward, and so they are most effective when the lumbar lordosis is already being maintained by the erector spinae and the iliopsoas muscles.
Figure 10.6. A generic meditative sitting posture (in this case the accomplished pose), with a right tetrahedron superimposed on the model. The base triangle rests on the ground, with its back corner underneath the coccyx. The middle upright line extends to the top of the head at a right angle (90°) from the plane (and back corner) of the base triangle.
The longus colli muscles on the anterior side of the cervical vertebrae act to increase lilt in the region of the head and neck. And like the erector spinae in the thoracic region, these muscles are situated on the convex side of their vertebrae (fig. 8.13). If the longus colli muscles along with the scalenes acting as synergists are not fulfilling their role in supporting tht posture, the head and neck droop to the rear.
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