Not sure if I'm on target with this, but I'll throw it out there and let you decide.
[url]http://www.performbetter.com/catalog/matriarch/OnePiecePage.asp_Q_PageID...

Check out these videos to get some help with the techinique.
[url]http://www.performbetter.com/catalog/matriarch/OnePiecePage.asp_Q_PageID...

If you use them they'll get turned back on. Do exercises that target the muscles and you'll re-educate them in the proper way to move. Strengthen them then perform the task you want them to perform once they're strong enough to do the task.

When you say muscles "shutting down" are you asking about this happening during the course of a race, or just concerns about not having the range of motion you once had.

If this happens during a race, I would say it is a nutrition/lactic acid build up issue. This is hard to recover from once it starts during a race, so the best plan here is to start with a better nutrition plan for the race. Lots of info out there on this.
Found this yesterday: http://67cshdocs.com/Nutrition/EnduranceDoc_Nutrition.htm

If you have concerns about not having the range of motion you once had, I would check out some of the Functional Movement type exercises mentioned here:
http://www.youtube.com/watch?v=lJYx2RnZgaw

I have been doing these for several months now and they have really seemed to take care of some weak areas. I had a bum hip that was bothering me after a marathon. Even after resting/swimming for a month, the pain was still there. I didn't know if I had a hip pointer, bursitis, arthritis, or some other itis. Anyway, I started doing the FMS workouts 2x per week, and the pain cleared up in 2 weeks and I never looked back.

I'm just an avg Joe, so take everything with a grain of salt. Hope this helps.