Marshall et al is a decent and recent example of evidence that stretching can improve flexibility. When subjects were stretched with the same force (torque) applied, pushed to the same level of discomfort, they could go 20% farther. So we know something changed! But was is it a “plastic” deformation in the tissue? Or a neurological change in tolerance for stretch? Despite plenty of contrary evidence, Team Plasticity remains large and devout.
Marshall et al followed their data into an overinterpretation in favour of plasticity. Because range increased, but pain at the end of the range did not, they unwisely concluded that a change in tolerance was probably not a factor. But stretching farther without hurting more could certainly mean an increase in tolerance! Although it wasn’t measured, it’s safe to assume the subjects’ pain would have been less if stretched only to the end of their original range.
The experiment was simply agnostic on the “how” question. It demonstrated only an increase in extensibility, not whether it was due to neural or structural adaptations. The pro-plastic opinion was just an opinion, and a good example of confirmation bias at work.