Musculotendon Modeling with Fully Cartesian Coordinates and a Generic Rigid Body

This work presents the steps required to characterize muscle contraction dynamics and implement musculotendon units (MTU) with a rigid tendon within the scope of a multibody formulation with fully Cartesian coordinates and a generic rigid body (FCC-GRB). The FCC-GRB emerges as an alternative formulation, gathering the main advantages usually associated with traditional global methodologies. Due to the nature of the coordinates utilized to describe the multibody and the strategy adopted to define rigid bodies, this formulation offers several advantages in terms of implementation and modeling procedures that makes it particularly suitable for biomechanical applications. Based on a Hill-type muscle representation, two different methods, one defining explicitly the MTU as a muscle actuator and the other implicitly as an external force, are presented. In order to validate the methodology, it is applied in the inverse dynamic analysis of three simplified lower limb test models. The obtained results were consistent with the analyzed movement, accurately predicting both concentric and eccentric contractions, and the action of biarticular and neutralizer muscles.