Validation of skeletal muscle models in multibody dynamics: A collaborative collection of benchmark cases

Abstract The definition of the musculotendon model is a critical step in the implementation of musculoskeletal models. However, the redundant nature of the muscle force-sharing problem and the diversity of functions available for modeling muscle parameters make the computational validation of new models and formulations a challenging aspect, as no standard reference data is available. The present work proposes a collaborative benchmark framework for validating musculoskeletal models implemented using a multibody dynamics formulation. Based on the use of simplistic biomechanical models, five benchmark cases are introduced to progressively guide the implementation and validation of muscle contraction dynamics and musculotendon models. Specifically, the first case validates the muscle contraction dynamics model. The second case focuses on modeling musculotendon units (MTUs) with via-points. The third case addresses the muscle force-sharing problem, while the fourth and fifth cases evaluate the ability of the implemented models to predict the neutralizing actions of antagonist muscles and to handle biarticular muscle modeling, respectively. In order to facilitate the implementation of the benchmark cases, this work details the steps required for developing a muscle contraction dynamics model, modeling MTUs within a multibody framework, and solving the muscle redundancy problem using static optimization. The proposed cases were solved using two distinct multibody formulations, namely Cartesian coordinates (CC) and fully Cartesian coordinates with a generic rigid body (FCC-GRB). The main MTU outcomes for each benchmark problem are presented, with detailed results provided as supplementary material. The proposed cases serve as an effective platform for the validation of skeletal muscle modeling methodologies.