la leche league high impact sport nursing softcup bra

You can combine jobbing or other high impact exercise with nursing.

Check this out: The La Leche League High Impact Softcup Sports Nursing Bra features a pull over design in cotton/spandex and waffle mesh to hold you comfortably during workouts. The drop cup makes for ease in feeding the baby with a one hand nursing buckle. It's available in white/grey and pink/grey and you even get a free nursing bracelet with your purchase - plus La Leche League International gets a donation.  

La Leche League also has a low impact soft cup sports nursing bra here.