This appears to be a journal article. Why should it have citations? This is an arena where people are supposed to be doing primary research. "I have a PhD and this is my opinion" is probably at least 20% of academics.
Primary research does not include simply stating “this is my opinion” - and this article also does not consist of someone stating their opinion, it is someone claiming to describe widespread behaviors and explanations for them.
There is a search bar right above this article if you want to look that research up. This is some physiotherapist recording his opinion; based on field experience. I'm going out on a limb and saying none of the other researchers in the field have been writing about his experience so there isn't anything to cite.