Classical yoga is presented as a scheme of eight components, and physical posture is one of them. Reading the list clarifies why contemporary practice differs so sharply from the source.
The first two limbs concern conduct
The scheme opens with restraints and observances, covering how a practitioner behaves towards others and how they regulate their own life.
These are placed first rather than added as context, and commentators treat the ordering as meaningful, on the view that later stages depend on the earlier ones.
Contemporary classes rarely address this material directly, which is a substantial omission relative to the text even where the physical instruction is careful.
Posture occupies a single item
The third limb concerns the seat, described in the classical source in very brief terms as steady and comfortable.
The text says almost nothing about specific shapes, and the elaborate posture repertoires that define modern practice belong to later and largely different literatures.
Its purpose in the scheme is preparatory, producing a body that can sit still long enough for the later stages to be attempted.
Breath and withdrawal form the transition
The fourth and fifth limbs concern regulation of breath and the drawing of attention away from external stimuli.
These sit between the outward-facing components and the inward ones, and the arrangement treats them as the bridge rather than as ends in themselves.
Both are described with cautions in the commentarial tradition, and the fourth in particular is presented as requiring a teacher rather than a manual.
The final three describe stages of concentration
The last limbs cover fixing attention, sustaining it, and a state in which the distinction between attention and its object is described as falling away.
These are presented as a continuum rather than three separate techniques, differing in degree of continuity rather than in method.
They are the destination the whole scheme is arranged towards, which makes the modern usage of the word yoga for physical exercise a considerable narrowing.
The scheme is one system among several
Other yoga literatures organise practice differently, with their own vocabularies, aims and lists, and some do not use the eightfold arrangement at all.
Presenting the eight limbs as the definition of yoga therefore privileges one text within a much larger and more varied body of material.
It remains the most widely cited framework in teacher training, which is why students encounter it first and often encounter nothing else.