What causes muscle contraction in times of stress? People say it's caused because of fight or flight response is activated, but this not exact. I am looking to find the direct cause, i.e is it a direct effect of adrenaline or maybe a direct effect of cortisol? Maybe is because glucose levels rise and it's the way of our body to use that extra energy? I don't really know and searching doesn't help.
Presumably you to mean to ask what pathway mediates muscle tension. muscle tension is controlled by the reticular formation (in the brain stem), so it doesn't require anything special to happen in the bloodstream. The reticular formation is connected to many other areas of the brain.
To save this question from triviality and relate it to your prior one (which was about anxiety), muscle tension is the most common (if not the defining) feature of GAD, yet muscle tension cannot be understood in terms of the autonomous nervous system (ANS) response to stress (according to a 2009 review of Pluess et al.) That's basically a coy of saying we don't know what exactly causes muscle tension in GAD. Furthermore, some studies of GAD reported that the typical ANS responses to stress are not present in many GAD patients (according to the aforementioned review citing in Hoehn-Saric et al.). The latter issue is more controversial because a later (and more cited) study of Thayer et al. did find some correlates with ANS in terms of cardiac activity, but this still doesn't quite provide a complete explanation. Another 2009 GAD review discussed 5 different conceptual models of GAD, yet none of these theories does much to explain muscle tension in GAD.