Muscle extensibility is influenced by its viscoelastic properties but this would depend on the tensile force applied.
Plastic deformation occurs once a stretch is sufficient to elongate the connective tissues within the muscle beyond its elastic limit and into the plastic region.
Once the stretching force is removed, the muscle would be unable to return to its original state and would remain in permanent elongation.
Increased Sarcomeres in Series
There is an increase in the number of sarcomeres in series, when muscles are immobilised in fully extended positions.
Although often reported otherwise, these muscles demonstrated no overall change in muscle length because increases in the number of sarcomeres in series are offset by a concurrent decrease in sarcomere length.
On the other hand, when muscles are immobilised in shortened positions, there is a decrease in the number of sarcomeres in series and a concurrent decrease in muscle length.
The most convincing reasoning to me is:
The end point of a stretch is highly subjected to sensation (pain onset, maximum stretch or maximum pain tolerated).
Reinforcing what I have said previously, I stand by the point that a short term stretch of 6 weeks is not sufficient to result an increase in muscle length.
The ability to reach more, be it flexing or extending, is mainly guarded by the sensation of stretch/pain.
To me, the feeling of pain and stretch is almost similar. For example, everyone have different pain thresholds and same goes to stretching. Some of our sensory fibres are more sensitive than others, therefore, the feeling of maximum stretch would differ.