Most people who have a sternalis muscle never know it. This extra muscle sits on the front of your chest, running vertically along your breastbone on top of the pectoralis major, and roughly 6% of people have one. It produces no symptoms, serves no known function, and is almost always discovered by accident during a mammogram, CT scan, or surgery.
So if you’re wondering whether you have one, the honest answer is: you probably can’t tell from the outside. But there are a few situations where this hidden anatomy makes itself known, and understanding what it is can save you real anxiety if it ever shows up on an imaging report.
What the Sternalis Muscle Actually Is
The sternalis is a thin, flat strip of muscle tissue that runs vertically (up and down) along one or both sides of the sternum, your breastbone. It sits right on top of the pectoralis major, the large chest muscle most people are familiar with. Unlike the pectoralis, which fans out horizontally toward your shoulder, the sternalis runs parallel to the sternum in a narrow band.
It has no known function in the body. Anatomists consider it either an accessory muscle or a vestigial leftover from development. It doesn’t contribute meaningfully to chest movement, breathing, or arm strength. You could have one your entire life and never notice a difference in how your body works. Its size, shape, and exact position vary widely from person to person, which is part of why it causes confusion when it does appear on imaging.
Who Is Most Likely to Have One
A large meta-analysis covering over 27,000 adults found a worldwide prevalence of about 6%. That means roughly 1 in 17 people has this muscle. Rates vary significantly by population. People of East Asian descent have the highest reported prevalence at around 9.1%, about three times the rate seen in populations of European or South Asian descent (around 3% to 4%). North and South American populations fall in a similar range, at roughly 3.4% to 3.7%.
The muscle can appear on just one side of the chest or on both sides. It occurs in both men and women, though it’s far more commonly discovered in women simply because mammography screening catches it incidentally.
Why You Can’t Feel It From the Outside
If you’re pressing on your chest trying to find it, you’re unlikely to succeed. The sternalis is typically a thin band of tissue, not a bulky muscle belly. It lies flat against the surface of the pectoralis, covered by skin and breast tissue (in both men and women). In most people who have one, it’s too small and too similar in texture to surrounding tissue to create a visible or palpable difference.
In rare cases, a very well-developed sternalis might create a subtle fullness or asymmetry near the inner edge of the chest, particularly in lean individuals with low body fat. But this would be easy to confuse with normal variation in pectoral muscle shape, and no reliable self-exam technique exists to identify it.
How It’s Actually Discovered
The sternalis muscle is almost always found through medical imaging. On a mammogram, it appears as a well-defined soft tissue density projected over the inner (medial) part of the breast, close to the sternum. It shows up on the standard top-to-bottom (craniocaudal) mammogram view as an extra structure that isn’t normally there. On CT or MRI scans, it’s identifiable as a longitudinal strip of tissue lying in front of the pectoralis major muscle, running along the sternal margin.
It can also be discovered during surgery. Surgeons performing mastectomies, breast augmentations, or other chest procedures occasionally encounter the muscle unexpectedly. Plastic surgeons have even found a practical use for it: the sternalis can serve as a tissue flap for reconstructive surgery of the chest, head, or neck.
Why It Gets Mistaken for Something Serious
This is the part that matters most if you’re reading this after a confusing mammogram result. Because the sternalis muscle is relatively rare and varies widely in shape, radiologists who aren’t familiar with it can mistake it for a concerning finding. On mammography, it can mimic a surprisingly long list of conditions: breast cancer, fat necrosis, abscess, hematoma, swollen lymph nodes, surgical scarring, and several types of benign tumors.
The muscle creates what radiologists call a “blind spot” on the inner side of the mammogram. It can appear as an irregular, dense structure that looks suspicious on initial review. In one reported case, a male patient’s mammogram showed symmetrical soft tissue masses projected over both breasts that turned out to be bilateral sternalis muscles.
The key to correct identification is recognizing the muscle’s characteristic shape and position. On CT or MRI, its longitudinal orientation and location directly over the pectoralis margin are distinctive. If a mammogram picks up something unusual near the inner edge of your breast, follow-up imaging with CT or MRI can typically confirm whether it’s a sternalis muscle rather than a mass that needs biopsy.
Can It Affect Your EKG?
Yes, in some cases. Because the sternalis muscle sits directly over the front of the chest wall, right where EKG electrodes are placed, it can alter the electrical signals recorded during a standard electrocardiogram. The extra muscle tissue between the electrodes and the heart may change the appearance of certain waveforms. This doesn’t mean anything is wrong with your heart. But if a doctor notices unusual EKG readings and you’re known to have a sternalis muscle, that context can prevent unnecessary cardiac workups.
What to Do If You Think You Have One
There’s no reason to seek out testing specifically to find a sternalis muscle. It’s harmless, painless, and functionally irrelevant. The only time it matters is when it shows up unexpectedly on imaging and needs to be correctly identified so it isn’t mistaken for disease.
If you’ve had a mammogram that flagged an unusual density near your breastbone, or a CT scan that mentioned an unexpected soft tissue finding along your sternum, a sternalis muscle is one possibility worth discussing with your radiologist. Cross-sectional imaging (CT or MRI) is the most reliable way to confirm its presence, since these scans show the muscle’s full length and its telltale position in front of the pectoralis major. Once identified, it requires no treatment, no monitoring, and no follow-up. It’s simply part of your anatomy.