Weeks of stretching produce a visible increase in range of motion, and the usual explanation is that the muscle has lengthened. Measurements of the tissue itself do not support that account, and the alternative explanation is more interesting.
Stretch is limited by a protective reflex
Muscles contain sensory organs called muscle spindles that report length and the rate at which length is changing.
When a muscle is stretched quickly, spindles trigger a reflex contraction in the same muscle. The reflex exists to protect against sudden overextension, and it operates below conscious control.
A stretch therefore meets resistance from the very tissue being stretched, and that resistance arrives before any structural limit is reached.
The end point is discomfort, not tissue failure
The position where a stretch stops is set by sensation. A person stops when the pull becomes uncomfortable, which happens well short of the point where tissue would be damaged.
That threshold is a perception generated by the nervous system, and perceptions can be revised by repeated exposure.
Studies that measure passive resistance in the tissue find it largely unchanged after training programmes, while the angle the person tolerates has increased noticeably.
Tolerance shifts before structure does
Repeated stretching in a controlled way teaches the nervous system that the position is not dangerous, and the reflex response is progressively reduced.
What changes is how much stretch the system will permit before generating a strong protective signal. The muscle has not grown longer; the permission has widened.
This is why range of motion can improve within a single session, far faster than any tissue could remodel.
Held postures and breathing work on the same system
Holding a position quietly for a period allows the initial reflex response to settle, which is why a stretch becomes easier thirty seconds in.
Slow breathing during a hold reduces general arousal, and a less aroused nervous system generates a weaker protective response.
The instruction to breathe into a stretch has a plain physiological reading, whatever else is attached to it.
Structural change is possible but slow and specific
Connective tissue and muscle can remodel with prolonged loading over months, and gains held for very long periods do involve some genuine adaptation.
Even then, joint shape and ligament arrangement place hard limits that no amount of practice moves, and those limits differ between people for reasons of anatomy.
Pain that is sharp rather than a broad pull, or that persists after a session, is a signal to stop and seek advice rather than to push further.