Stretch marks are caused by tearing in the skin/connective tissue, either by direct trauma or by "stretching" due to enlargement of tissue, such as muscle, underlying the skin and tissue immediately under the skin. There are three layers of skin: the epidermis, or outer layer; the dermis (the elastic, resilient middle layer), and the subcutaneous tissue, the deepest layer. Stretch marks (also known as stria atrophica and striae distensae or striae gravidarum reflecting their occurrence in pregnancy) form in the dermis. When the dermis is stretched, the connective fibers break, there is microscopic bleeding and tissue inflammation, and healing occurs with scar formation. The result is the markings we know as stretch marks. Stretch marks begin as raised pink, reddish brown or dark brown lines that then turn a brighter violet or purple. Gradually these bright marks flatten and fade to a color a few shades lighter than your natural skin tone. They usually become less noticeable over time.

Stretch marks can appear anywhere on the body where the skin has been stretched (often as a result of pregnancy, weight gain, muscle enlargement secondary to weight training, or direct injury such as occurs with repetitive weight training exercises). They're most likely to appear in places where fat is stored: the abdomen, breasts, upper arms, thighs and buttocks. They pose absolutely no health risk and don't compromise your body's ability to function healthily; they are purely a cosmetic issue and need to be treated accordingly.

There have been several treatments proposed, but none are really effective. Skin massage with or without moisturizing cream may be tried, a diet containing foods that are thought to contribute to the overall health of the skin, such as those high in vitamins C and E, zinc and silica (which helps to form collagen) have been suggested. There are a large number of commercial products sold for the treatment of stretch marks, none of which, to my knowledge, is particularly successful. A prescription product, Retin-A, which is a derivative of Vitamin A that is applied to and rubbed into the skin, has actually been shown to reduce the prominence of stretch marks, but it is expensive, does require a prescription, and must not be used on women who are or may become pregnant because it has very damaging effects on a developing fetus.

Prevention is problematic for body builders. Avoiding large numbers of repetitions of the same lift may help, but that may interfere with your body building goals. Do understand that, over time, the stretch marks will become less obvious as they heal, but that they will never entirely disappear once they form.