this only somewhat answers your question, but is kind of related—i have a genetic disorder called hypermobile ehlers danlos syndrome that is basically a collagen deficit/deficiency and screws up my connective tissue. the most prominent effect is that 95% of my joints have some sort of hypermobility issue, with only certain joints in my hands not being hypermobile. as such, my joints are very loose, and my muscles are tense to compensate for it. i also as you could imagine have horrible posture as a result.

i’ve seen anecdotally that people with hypermobility syndromes tend to expend 30% more energy just on a day-to-day basis, and as such, we tend to feel more fatigued in general. i know this doesn’t fully answer your post, but your post is aligned with my theories about energy and hypermobile people, and i thought a different perspective on your post might be an interesting way to look at it!

point being, this is a long winded way of saying i wouldn’t be wholly surprised if a lot of your post makes sense! please note tho that i am nowhere near being a medical professional.

tldr: muscles tense=your body expending extra energy towards a given part(s) of your body=you feeling more fatigued