It's hard to describe this. I'm 33. I imagine that muscles strain more easily with age, but what about these instances. I've noticed sometimes that if I stretch the bottom of my left cheek/mouth out, the muscles between that area and my neck get momentarily stuck in place. If I stretch my toes downward (not with my hands, just my feet) they also get momentarily stuck in place and there's a momentary pain cause the muscles lock up. Then it subsides.. It's similar to the feeling of the abdominal muscles straining after too much exercises. I'm a bit of a hyponchondriac but I'm curious about this because it didn't happen when I was younger and it feels like I could accidentally pull my muscles very easily.
Did you mean to post on the Polymyalgia board? From what you say it sounds like your symptoms are from something else.