Wow, reading all these is amazing.
Sitting for me is the absolute worst. And the worst of the sitting is driving, followed closely by typing and sitting on a plane. So bad that for 3 years i did not go to a movie, I had to move to be close to work, and didn't go on a car ride longer than 15 to 20 minutes for years. Now, after years of training, I can sit for extended periods, but i'll pay a price the next few days. After a bout of sitting, my shoulders will hunch, core disengage, hip flexors tighten, neck and lower back stiffen, pecks tighten, and i'll be stiff for between 1 to 3 weeks.
The thing that absoultely has helped me the most is finding a kinesiologist who understands the importance of correct form and not overdoing it. Literally it has taken years to recruit the right muscle groups to stabilize myself appropriately. For me, what I learned is that my totally lax joints and the HMS have had a number of very intertwined outcomes and resulting consequences.
1. the lack of joint stability has contributed to overwork by certain muscle groups (prematurely).
2. The development and dominance of certain muscle groups (lower back, hip flexors, pecks) for stability has unfortunately arrested and disrupted the neuromuscular development of other groups, including quads, glutes, lower traps, core.
3. My muscles get tired easier, and take longer to recover, than most of the people i know without HMS. What that has also meant is that my muscles are weaker and have less endurance than I (and my PE teachers) had expected them to have. Early fatigue contributes to compensatory muslces (back, neck) to do extra work when it's really not there intended purpose.
4. Over time, the dominant muscles get fatigued, chronically tense, knotted, and cause stiffness and pain.
The key with working with my trainer has been:
1. attention and development of the under-developed muscle groups.
2. reduced attention to the already overworked dominant muscle groups.
That is, instead of stretching the hamstrings and hip flexors, we'll spend most of the time isolating the core (with leg lifts), glutes (w side leg raises) and lower traps (with rows), with lot of tactile queing to make sure the right groups get turned. There is nothing that helps my stiffness more than working out the antagonist muscle groups.
I also get trigger point massages frequently, watch my stress level, and am in psychotherapy which all telp a ton.