Abstract:
This paper presents a new framework for creating three-dimensional (3D) finite-element (FE) models of skeletal muscles to overcome the limitations of traditional models that represent muscles as simple line segments. The goal was to develop a method that could accurately capture complex muscle geometries, the intricate arrangement of internal muscle fibers (muscle architecture), and the resulting variation in function across different parts of a muscle. Using magnetic resonance (MR) images, the researchers created detailed 3D FE models of four hip muscles: the psoas, iliacus, gluteus maximus, and gluteus medius.
A key innovation was the development of "template fiber geometries" that could be mathematically mapped onto each muscle's unique shape to represent its internal fiber structure. The muscle tissue itself was modeled using a transversely-isotropic, hyperelastic constitutive model to account for its fiber-reinforced mechanical properties. When the models were subjected to simulated hip joint motion, they revealed a substantial variation in moment arms among fibers within each muscle. For example, the peak hip extension moment arm for fibers within the gluteus maximus ranged from 1 cm to 7 cm. This finding challenges the assumption made in simpler models that all fibers within a muscle have the same mechanical action. The models were validated by showing that the predicted changes in muscle shape during hip flexion closely matched shapes observed in MR images, with average errors of only 1.7 to 5.2 mm. This new modeling approach enhances the biomechanical accuracy of musculoskeletal simulations.
