It appears that one can often talk about the effects of Yoga Āsana on the spine in Yoga, yet the reality is more based on the effects of Yoga Āsana on the external aspects of the structural form. It has also been an observation over some four decades of teaching Yoga that the two can get confused in terms of assessing developmental progress within the practice of Yoga Āsana.
Furthermore it appears that it is possible to work the body into ‘advanced’ Yoga Āsana yet observe that the spine is not deeply influenced, for example with the hips and shoulders or lax joint ligaments facilitating the impression of the form. Hence the application of Yoga from this perspective is to start with the spine as the primary priority with the limbs the secondary priority.
Thus the principles of modification of Yoga Āsana are from the perspective of allowing adjustments to the limbs in order to facilitate a deeper more profound impact on the spine.
For example it is possible to perform Yoga Āsana such as Uttānāsana with the spine relatively inert or even fixed into a lumbar lordosis with the impression of movement being conveyed through the range of movement manifesting via the hips, buttocks and hamstrings.
This means that the starting point for any observation within Āsana needs to include what is happening in the spine. This is an initial and essential reference to be satisfied, then the work can shift to include other areas as part of the focus. However the reference for whatever you choose to include as a focus that moves the attention away from the spine is to bee evaluated by the affect, one way or another, on the spine.
These are part of the criteria that Desikachar taught from around the skills to assess the difference between movement in the body and movement in the spine. Woven into this primary teaching and understanding around observation in Āsana, are also the energetic processes inherent in Haṭha Yoga.
These principles, to be applied along with those on the physiological priorities around the spine, can also become neglected within interest in ‘contemporary’ body based paradigms increasingly overlaid onto and into Yoga Āsana from such as exercise, movement, dance or somatic based sources.
Hence Desikachar taught me that it is more vital to observe, from both a structural and energetic perspective, how Yoga Āsana influence the spine rather than the more generalised impression of how Yoga Āsana influence the body. In other words assessing the effect of the Yoga Āsana from the inside out rather than the more usual impression arising from observing from the outside in.