It's the perfessers again!

I clearly should have pursued a career in science - Jogging your way to saggy breasts:

Some 9.5 million British women could be irreversibly damaging their busts by exercising without a proper sports bra, the Portsmouth University team said.

They found breasts moved in a 3D figure of eight and that uncontrolled movement strained fragile tissues and ligaments.

The study suggested as a woman runs a mile, her breasts bounced 135 meters.

The report found each breast moved independently of the body by an average of 9cm for every step taken on the treadmill.

With the average breast weighing between 200 and 300 grams, this movement puts great stress on the breast's fragile support structure - the outer skin and connective tissues known as Cooper's ligaments.
...
They claim that wearing an ordinary T-shirt bra reduced bounce by 38%, but wearing a sports bra by the firm Shock Absorber - which sponsored the research - reduced bounce by 78%.

Not to mention a twin I-beam suspension.