Several studies have shown that regular exercise reduces the rate of breast cancer (by about 30%). This might be connected to increased breast movement. Most of these women wore bras while exercising, and it is interesting to ponder whether brafree exercising would have produced even better results. Is brafree walking as good for the breasts as brabound running? The question is "how much movement?" Does subtle movement help? Even in the absence of dramatic breast movement while walking, the breasts do wobble and deform somewhat which would help in pumping lymph. Perhaps brafree walking is enough to aid in this pumping action. Perhaps brisk walking is much better. Unfortunately, until the bra/disease connection is taken seriously, we will not know the answers.
One player in this might be the retromammary space. This is an area of loose connective tissue between the breast and the underlying chest muscles, which allows somewhat free breast movement on the chest wall. Some people have spoken of a pumping action whereby the movement of the breast helps aid lymphatic circulation. As Debra Curties, president Sutherland-Chan School of massage in Toronto, states, "The retromammary space also plays an important role in the lymphatic drainage of the breast." Perhaps the breast movement that sports bras prevent so well is a vital part of breast health.
Women do a bit of judging for themselves the effects of various forms of brafree exercise and/or massage on things like overall comfort or monthly breast fullness. If a certain type of exercise helps fullness, it is probabaly helping circulation.
see also: http://massagetherapy.com/articles/index.php?article_id=547