Millions of women may be jogging their way to sagging breasts as they set off on New Year fitness regimes without suitable bras, research suggests.
Anna Kournikova led a campaign to get women wearing sports bras
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 135m.
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.
Irreversible breast sag
The research team found breast movements resulted in temporary pain and discomfort.
But it also led to a more permanent stretching of the Cooper's ligament which leads to irreversible breast sag.
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%.
Dr Joanne Scurr who carried out the research said women needed to be aware of the effects of exercising without properly supporting their breasts.
"People rightly spend time and effort to get the right footwear for exercise and it is equally important to wear the correct sports bra.
"Proper support for breasts will reduce the stretch to the Cooper's ligament."