while I understand what's being said about the causes of myopia related to length of eyeball etc, I do think there's two differences in causes of these conditions, one I (as a layman and reading the above) believe to be physiological and one to be environmental.
I can't say and have never been told by an optician (don't know if they can) whether my fault is through these cillary muscles or a change in eyball shape/length.
But I sit a computer all day, and until recently all while wearing glasses for distance. This must have a detrimental effect on eyesight surely as I know the position I take at my desk caused a minute lengthening of the muscle across the back, which delayed the healing of a shoulder injury due to alignment.
So why not the eye also??