Why many women have sour muscles between the shoulder blades?

T5 is the point of maximum curvature in the upper back and is located between the shoulder blades. A lot of people hunch their head and shoulders forward a great deal, which obliges many muscles in the upper back to work overtime. It's as though the head and neck are cantilevered forward from T5 on up. This is the most common reason for people to be sore there. I like your expression "having sour muscles." The Chinese use the description of "sour" for this sensation as well. It makes sense as it refers to lactic acid build up or release in the muscles. It just isn't used commonly in English. If you try to hold your shoulders back (an incorrect way that people use to try to have good posture), that will overwork the rhomboids and their insertions in the thoracic spine can get sore, but it is not that common for people to try to have good posture these days (and if this is the way they go about it, it's just as well)...

Maybe you mean [i]sore[/i]?  :) I'm a massage therapist and have many thoughts on this topic. I think some women (especially those who developed early or are large breasted) protract their shoulders and hold their heads forward, which minimizes the appearance of their breasts, to avoid unwanted attention.  This forward-shoulder posture puts the muscles in between the shoulder blades on stretch making them sour/sore. I think  also, that the sheer weight of breasts can contribute to too much kyphosis.  This also causes the shoulders to protract, putting the muscles between the shoulder blades on stretch.