"Women lose up to four centimeters of stride length due to poorly controlled breast movement while running—which, over the length of a marathon, could translate to running an extra mile, according to a recent study conducted by Brooks Running Co. in conjunction with the University of Portsmouth."
Sports bras have come a long way! I highly recommend finding a good one, it can make or break a workout in some instances. Breasts do not simply move up and down, they also move forward and sideways simultaneously.
There have been some studies since the 'jock bra' was created in 1977 as the answer to breast motion during exercise. The overall take away from these studies is that excessive breast motion during physical activity causes discomfort (duh), but also embarrassment.
In 2009 there was a study by done on running mechanics, 18–40 year old women, that were currently active (exercised aerobically at least once per week), were not pregnant, had not given birth or breast-fed in the last year, were a D-cup breast size and had no previous
surgery to the breasts. The findings of this study suggest that inadequate breast support affects a female's running kinetics, which may have negative physiological consequences on sports performance.
So if it's affecting our quality of movement, and our quality of movement effects our daily life, it's about time we got this right!