Hyper-elastic properties of the human sternocleidomastoideus muscle in tension

Numerical models of the human body require realistic mechanical properties of the muscles as input, but, generally, such data are available only for animals' muscles. As a consequence, the aim of this study was to identify the hyper-elastic behavior of the human sternocleidomastoideus muscle in tension using different constitutive laws. Ten sternocleidomastoideus muscles were tested in vitro. The hyper-elastic behavior was modeled with an exponential law and a hyper-elastic constitutive law studied analytically. The latter was also studied with an inverse approach using a subject-specific, finite-element model of each muscle. The three approaches were compared statistically. From these laws and methods, the shear modulus μ (4 to 98 kPa) and the curvature parameter α (17 to 52) were identified. Both the analytical and finite-element approaches gave parameters of the same order of magnitude. The parameters of the exponential and hyper-elastic laws were linked thanks to simple linear equations. Our results evidence that the hyper-elastic tension behavior of human sternocleidomastoideus muscle can be described using a simple model (exponential) considering basic geometric features (initial length and cross-sectional area).