Well, the word back from lactnetters is that some of us just feel a tugging
when we run without a bra, others feel a real discomfort, and others feel
outright pain.  For most of us, walking is okay;  the problem comes with
things like running.  (Does that mean we're not biologically designed to
run?  Fine with me!)

The amount of discomfort varies with stage of lactation/time of month, of
course, but yes, there are those of us who wear bras to prevent physical
discomfort/pain, not just to stop the jiggles.

Diane Wiessinger, MS, IBCLC, LLLL  Ithaca, NY