Breast tissue contains no muscle. It is supported entirely by skin and Cooper's ligaments — fibrous bands of tissue that connect the breast to the chest wall.
How they stretch
Cooper's ligaments stretch from gravity, weight changes, pregnancy, and especially repetitive bouncing during exercise. Once stretched, they do not return to their original length.
Why this matters for bra fit
A supportive everyday bra and a properly fitting sports bra protect the ligaments. This is not vanity — it is long-term breast tissue health.