Here's an article on the movement of breasts during running and I believe this holds true when doing high impact.
Note: this research was sponsored by a bra manufacturer but I absolutely do believe you need a good (sports)bra
http://news.bbc.co.uk/2/hi/health/4607642.stm
Note: this research was sponsored by a bra manufacturer but I absolutely do believe you need a good (sports)bra
http://news.bbc.co.uk/2/hi/health/4607642.stm