Traditional bras are breast focussed and fix to the chest wall, tightening bra straps pulls the shoulders down and underwires push into the ribs. This creates an imbalance in posture, which then sets up a domino effect throughout the body, resulting in neck, shoulder and back pain.
At the same time this also puts pressure on the vertebrae at the top of the back and the bottom of the neck (vertebrae C6 and C7). This pressure can cause a thick, dull migraine type headache across the forehead and above the eyes. Recent research has shown that removing breast weight from the shoulders results in a dramatic relief of symptoms.
For more information, see: Ryan EL pectoral girdle myalgia in women: a 5-year study in a clinical setting. Clinical Journal of Pain 2000 Dec.