The fascicular anatomy and peak force capabilities of the sternocleidomastoid muscle

The fascicular morphology of the sternocleidomastoid (SCM) is not well described in modern anatomical texts, and the biomechanical forces it exerts on individual cervical motion segments are not known. The purpose of this study is to investigate the fascicular anatomy and peak force capabilities of the SCM combining traditional dissection and modern imaging. This study is comprised of three parts: Dissection, magnetic resonance imaging (MRI) and biomechanical modelling. Dissection was performed on six embalmed cadavers: three males of age 73-74 years and three females of age 63-93 years. The fascicular arrangement and morphologic data were recorded. MRIs were performed on six young, healthy volunteers: three males of age 24-37 and three females of age 26-28. In vivo volumes of the SCM were calculated using the Cavalieri method. Modelling of the SCM was performed on five sets of computed tomography (CT) scans. This mapped the fascicular arrangement of the SCM with relation to the cervical motion segments, and used volume data from the MRIs to calculate realistic peak force capabilities. Dissection showed the SCM has four parts; sterno-mastoid, sterno-occipital, cleido-mastoid and cleido-occipital portions. Force modelling shows that peak torque capacity of the SCM is higher at lower cervical levels, and minimal at higher levels. Peak shear forces are higher in the lower cervical spine, while compression is consistent throughout. The four-part SCM is capable of producing forces that vary across the cervical motion segments. The implications of these findings are discussed with reference to models of neck muscle function and dysfunction.