The Sternalis muscle has been nominated as a supernumerary anatomical variation; the highest prevalence is in China (23.5%) and the lowest in Chile (0.87%).

The muscle is a wide band of muscular tissue located in the anterior thoracic wall, where fibers travel near-parallel to the sternum, inferior to the clavicles, aterior to the pectoralis major and the pectoral fascia. It has received many names: rectus sternalis, parasternal, episternal, superficial abdominal rectus, among other names. To avoid confusion, a true sternalis muscle should have the following characteristics:

(1) be localized between the subcutaneous tissue of the anterior thoracic region and fascia pectoralis;
(2) originate superiorly from the sternum or infraclavicular region;
(3) be inserted inferiorly in the ribs, rib cartilage, external oblique muscle aponeurosis and sheath of the rectus abdominis muscle;
(4) its innervation must come from the anterior root of the intercostal nerves. Another possible innervation from medial and lateral pectoral nerves has been mentioned.

Even though the Sternalis muscle does not appear to have a significant function, it is important to bear it in mind when evaluating mammograms, because one could misdiagnose it as a mammary cancer, as exposed by many authors. It is crucial to be familiar with this muscle to avoid confusion with pathology, such as extra-abdominal desmoid tumors, diabetic mastopathy, abscesses, hematomas, fat necrosis, and others; its unilateral presence may cause breast or chest asymmetry.

Sources:
1. Jelev L., Georgiev G., Surchev L. (2001) The sternalis muscle in the Bulgarian population: classification of sternales. J. Anat. 199: 359-363. 
2. Bradley F.M., Hoover H.C., Hulka C.A., Whitman G.J., McCarthy K.A., Hall D.A., Moore R., Kopans D.B. (1996) The sternalis muscle: an unusual normal finding seen on mammography. AJR. Am. J. Roentgenol. 166: 33-36. 
3. Scott-Conner C.E.H., Al-Jurf A.S. (2002) The sternalis muscle. Clin. Anat. 15: 67-69. 
4. Shiotani M., Higuchi T., Yoshimura N., Kiguchi T., Takahashi N., Maeda H., Aoyama H. (2012) The sternalis muscle: radiologic findings on MDCT. Jpn. J. Radiol. 30: 729-734.