A finite element muscle building block derived from 3D ultrasound: application to the human gastrocnemius

Abstract

Muscle fascicle structure characterises muscle function, which in turn plays a key role in computer simulation of muscle shape. In this study, we use 3D ultrasound from the human medial gastrocnemius muscle to identify and map the muscle fascicle orientation and deformation during passive motion in four subjects. This muscle fascicle description is integrated into a representative muscle volume element using a free-form deformation technique to create a muscle primitive building block that deforms according to the embedded muscle fascicles within. For each subject computed passive tensile force was used to optimise the constitutive behaviour so that the known deformation matched this load. Each subject was fit to match deformation at 25, 50, 75 and 100% of muscle stretch. The subjects that exhibited a larger muscle fascicle pennation angle change during passive stretch were characterised best by a less stiff constitutive law. In contrast, subjects that had a more parallel muscle fascicle deformation, showed stiffer behaviour. The medial gastrocnemius muscle built from these muscle primitives exhibited a contractile shape that is consistent to that observed in human gastrocnemius contraction. This shape was evaluated against the same muscle embedded with muscle fascicles derived from diffusion tensor magnetic resonance imaging and was in good qualitative agreement. Muscle principal strain was shown to align with muscle fascicle direction and was spatially non-uniform. Muscle primitives may be used as building blocks to build large muscle volumes for mechanics simulation, visualisation and medical education.