The breasts are not muscle controlled flesh, like most parts of the body, but are delicate glandular structures entirely without muscle.
It is for this reason that it is important to keep them in good shape. The mammary glands are heavy, and have a natural tendency to sag. To support them, a well-fitted bra is essential, and every woman should wear a bra from the first development of the bust.
The more active or athletic a girl is, the more important it is to protect her bustline by supporting it firmly and comfortably in its correct shape and position. The breast should never be allowed to bounce, because this can stretch the skin and tissue and will cause increased sag to the bust. This in turn causes the breast to lose their shape.
Later in life, if the bustline has sotened a bra is even more essential, in order to lift the breast back to their correct position, relieves strain and discomfort and remould a natural shapely line.
*Taken from Triumph factsheet