Viewpoint: The Embryological Development of Sternalis Muscle and Implications for Trigger Point Pain

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Laurie Yew-Kin Hung

Keywords

embryology, myofascial trigger point, Pectoral nerves, intercostal nerves, pectoralis muscles

Abstract

Sternalis muscle (SM) is a recognized variant muscle of the anterior thoracic wall. Although the morphology of SM is agreed upon, the function is unknown and the innervation and embryological origin of this muscle are areas of debate. No existing theories regarding the origin of sternalis muscle explains the variability in innervation that sternalis muscle presents with (i.e. anterior branches of 1st-5th intercostal nerves vs. pectoral nerves). Most old and modern anatomists argue that SM is innervated by either the pectoral nerves (lateral or medial or both) or the 1st-5th intercostal nerves (anterior branches). It is also argued that SM is derived from either the pectoralis major muscle or the rectus abdominis. Several manual therapy books have proposed trigger point pain referral patterns based on innervation from both nerves (pectoral and intercostal). The authors of this paper propose that there are two types of SM, resolving apparent disputes in the SM focused literature, and offer an explanation of existing composite pain referral patterns of SM based on innervation.