[Musculoskeletal biomechanical analysis of the three-dimensional gastrocnemius and soleus muscles]

Musculoskeletal models based on multibody dynamics have been widely applied in human biomechanics research; however, conventional one-dimensional (1D) muscle models are limited in their ability to represent realistic muscle geometry and fiber architecture, resulting in restricted biomechanical predictions. Although three-dimensional (3D) finite element (FE) muscle models can provide more detailed mechanical information, high computational cost limits their application at the whole lower-limb scale. To address these limitations, this study develops a unilateral lower-limb musculoskeletal model with hybrid muscles based on the TLEM2.0 database, in which the soleus-gastrocnemius muscle group was represented using 3D FE models, while the remaining muscles were modeled using 1D Hill-type formulations. By coupling FE analysis with muscle recruitment optimization, muscle activation patterns of the right lower limb during the stance phase of level walking were obtained. Model predictions were validated against in vivo knee joint reaction forces and surface electromyography (sEMG) data. The results demonstrated good agreement between the predicted and experimentally measured knee joint reaction forces, and the predicted force patterns of the medial and lateral gastrocnemius muscles corresponded well with sEMG trends. In addition, the model enabled detailed observation of muscle deformation as well as internal tensile stress and strain distributions in the 3D muscles during gait. This hybrid musculoskeletal modeling framework achieves a balance between computational efficiency and biomechanical fidelity, providing an effective platform for investigating lower-limb muscle biomechanics.