I am now in my 39th nonconsecutive month of breastfeeding a baby and only now have I finally found a nursing bra for high-impact activities, such as running!
Any "sports" bra I've tried from Motherhood (maternity store) has been woefully nonathletic with one exception that appears to have been discontinued and I think that one only really works because it is a size too small.
I've tried the regular sports bras with a zipper on the front for "easy access", but it also leads to easy chafing.
I've even tried the one nursing bra offered by Title 9 Sports, but its stated use is for low-impact activities and they are right.
But it turns out that La Leche makes bras and they are great. I don't know whether it had been there all along and I just found it or if it is a relatively new item, but I highly recommend it. I wore it for the second time today on an 11 mile run with a break at the playground in the middle which included nursing the baby before heading home. It was very comfortable and there wasn't any chafing. It is available elsewhere, but was least expensive at Expressiva Nursingwear.
I considered including a picture - of the model, not me - but thought that might be a little weird.