The metaphorical model and the physiological model do not need to be in conflict.
They often are, and when they are it is the body that pays the price not the imagery.
A particular model such as the one you describe will have its corresponding practice.
Another model, still metaphoric, will have different associations.
By putting the waist between the hips and the shoulders, which happens to be the usual location for the 'waist', ie. the narrower part of the torso above the hips and below the rib cage, the body is divided at that part and your roll back is done by turning the upper body relative to the lower.
When I put the 'waist' at the hip joint, and divide the body into legs,torso(hips to shoulders, and arms, my roll back is done with the hip rotator muscles at the hip joint. There is not turning of the upper torso relative to the lower, both turn at the same time.
The psoas muscles are anchored to the anterior/lateral portion of the entire lower spine. They separate from the spine at the 5th lumbar vertebrae and transverse through the pelvic girdle and insert to the inside of the thigh bone, the femur.
They, along with the ilios are the hip FLEXORS, the group of muscles that are the prime movers when you lift the thigh to the trunk or when you bend the trunk to the thigh.
As a quick aside, the farther in front of the AXIS of the HIP JOINT the upper body is, and the more extended the femur is behind the axis of the hip joint, THE MORE THE POSTURAL CONTRACTION OF THE HIP FLEXORS.
This is why leaning tenses up the lower back and why leaning is to be avoided. This beside the fact that leaning shifts the center of gravity relative to the hip joint which also functions as the fulcrum of a lever. And so allows one to be easily toppled by an opponent who knows when body is unstable.
The psoas muscle is not used for rotation along the vertical axis but for rotation at the horizontal axis of the hip joints.
As a matter of fact, it helps to prevent the lumber spine from twisting.
The spinal rotation begins at the Lumbar 5 and THORACIC 12 vertabrae, above the lumbar spine.
There is a difference beween hip flexion, the decrease of the angle between the femur and the pelvis, and spinal flexion. Spinal flexion is in fact simply the removal of the lumbar curve. Same word, "flexion", but different meanings with respect to different parts of the body.
There are two issues, one is the matter of explosive torque, properly named ballistic movement.
The second is the issue of spinal ROTATION as such, so as to take the knowledge of how the spine moves and harmonize this with the metaphors, wheels,axles,scales, turning, etc. on which prescriptions of movement are based in Tai Chi.
Specific interpretations of the metaphors, eg if the waist is like an AXLE, result in very different movements.
If the axle is placed at the usual position called the waist, then you have the type of turning Jerry describes.
If the waist is placed at the hip-joint, you have an actual AXLE. There are different kinds of axles. The femur-hip articulation in one of its functions constitues a lever.
The leg is the base, the joint is the fulcrum, the hip, even though it doesn't look like the AXLE of a lever is one.
Any elementary text on bio-mechanics will have a full account of this because it is so important in all kinds of athletic activities.
Microtrauma is as important as ballistic trauma. Microtrauma is insidious and non-reversible in many instances.
I am aware that you care about these kinds of issues.
Because Tai chi movement are done slowly there is a belief that no damage results.
One reason for during thoracic rotation slowly is to not exceed the person's ROM.
At the first felt tension the movement should halt.
Those who believe that going beyond the tension while it is still active will end up inflicting damage while in pursuit of well being.
It is this sort of counterfinality, achieving opposite results to what is intended, that is important to deal with and prevent.
Any movement that results in applying ROTATIONAL movement to the lumbar spine, torque, is dangerous.
Yes, flexion/extension of the lumbar spine also requires care. That is another important issue besides spinal ROTATION.
"I would make the point that knowledge of anatomy and physiology does not guarantee kinesiological knowledge or kinesthetic awareness, which can more successfully be gained through training and experience."
Precisely. Knowledge of anatomy does not in itself give knowledge of human movement. For example, the topic of human locomotion is a hot topic in kinesiology. Lots of disagreemens about the role of the legs and the trunk in walking.
But knowledge of structure needs to be married to knowledge of function. This is precisely what kinesiology does with respect to parts of anatomy that have to do with human movement.
Kinesthetic awareness is relevant to the prevention of injuries. Yes, it can be trained, and is a part of kinesiology so as to apply general knowledge about range of movements to individual persons and their personal range of movement.
A unity of knowledge of general anatomy relevant to skeletal movement and proprioceptive kinesthesis is what is needed from the start.
It is this sort of knowledge that is ignored by many athletes including people doing Tai Chi.
If kinesthetic awareness so as as correct or prevent damaging movement is to be acquired through training and experience when this involves doing the movements without knowledge of what is happening, then often the knowledge comes too late. Comes after the damage is done and often is irreversible.
[This message has been edited by RonKreshmar (edited 05-13-2003).]