Tight hip flexor musculature causes excessive anterior tilt of the pelvis… hyperlordosis of the lumbar spine… hyperkyphosis of the thoracic spine… hypolordosis of the lower neck with hyperextension of the head at the atlanto-occipital joint… forward head carriage… tight posterior neck muscles… headache.
That people aged 18-21 in this study have not yet begun to experience pain as a result of text neck posture does not surprise me at all. They are simply still too young to experience what the physical stress of the overuse of text neck posture will eventually do to them.
The psoas major’s sagittal plane lumbar spine joint action is by far its most controversial function. The term “psoas paradox” describes this controversy.
There is an old saying that no posture is bad unless you get stuck in it. The problem is that the thoracic spine often does get stuck in bad posture.