Development of a three-dimensional model of structural and functional changes during skeletal muscle growth: Experiment, simulation and validation

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During growth muscle tissue is subjected to extensive structural and mechanical changes. Thereby, the increase of muscle force is related to the adaptation of the three-dimensional muscle geometry and the material behaviour of the muscle. The whole growth process happens while retaining the muscle function. The essential condition for the understanding and the modelling of growth is the acquisition of the structural and mechanical changes. Aim of the project is the development and validation of a growth model for skeletal muscles. This aim is ensured by two interacting steps, that are characterised by a tight complexity between experiment and modelling. The first project phase deals with the development of a growth model for the simple structured rabbit m. soleus. Rooted on experiments, we identify at 10 different animal ages (between the 4th and the 20th week of life) specific muscle parameters. This experimental work comprises the determination of the active muscle characteristics, the passive tissue behaviour, the muscle deformation during contraction, the three-dimensional muscle architecture, as well as the immunohistologic analysis of the muscle tissue. Based on these experimental results, growth kinetics for various state variables (e.g. pennation angle, tissue stiffness, fibre typ) will be identified, which will be used in a subsequent step for the development of the growth models (for muscle, aponeuroses, and tendon tissue). The model validation occurs on experimental force and deformation data that have been imposed from dynamic muscle contraction experiments for all muscle ages. In a second project phase the growth model will be verified on the much more complex m. plantaris. This multi-pennate muscle is characterised by a complex fibre architecture combined with inner tendon sheaths as well as an inhomogeneous fibre type distribution. A successful development and validation of the growth models as planed in this project would accelerate the knowledge concerning the growth behaviour of skeletal muscles in a distinct way. The knowledge of structural changes and contractile characteristics is a crucial requirement for a deeper understanding of muscle shape changes and the differentiation during growth. An adequate growth model allows the studying of state variables inside the muscle (e.g. inner pressure, stresses, fibre orientation) as well as their change during growth. This could lead to a better understanding of biomechanical relations. Furthermore, the successful development of growth models could be a basis to answer open evolutionary-biological questions like the three-dimensional arrangement (assembly of muscles in muscle packages) and the growth of extremities.