You started an intensive weightlifting program to pack on pounds of muscle in a month. Halfway through though you notice red, sometimes purple, lines streaking across your pecs and at the apex of your armpit and biceps. It's not an allergy to the gym or a rash—it's stretch marks.
What is a stretch mark?
"A stretch mark is really an absence—basically a hole in your dermis," says Michael Swann, MD, a dermatologist in Springfield, MO, who specializes in skin cancer, reconstruction, and cosmetic procedural dermatology. Think of a balloon. When you blow it up or stretch it, at some point before the balloon rips apart you’re going to see the material fade in color, get super thin, and bubble or ripple," Swann explains. That’s what your skin does.
Stretch marks are weaknesses in your skin because too much tension has been placed on it for too long. "Our skin is living and active, so it’s constantly responding, so when you put stretch on it, it grows; but it can only do so much before it can't keep up.
It's not a scar, though. A scar is a healing response, Swann explains. That's your body providing extra collagen to bolster a weak spot or wound.