Determining a musculoskeletal system's pre-stretched state using continuum-mechanical forward modelling and joint range optimization

The biomechanics of a musculoskeletal joint system is a complex interaction of three-dimensional, inhomogeneous, anisotropic and contracting muscle tissue with nonlinear mechanical behaviour. The human-specific range of motion (RoM) of a joint system is efficiently balanced by the pre-stretch of the individual muscles, which utmost effects their contraction capability. Using a forward approach for movement analysis of the musculoskeletal system of the forearm, we developed a novel optimization process to determine the muscle fibre pre-stretch situation for a given initial arm position. We created a detailed 3D finite element model of the forearm with the flexors \biceps, \brachi, and \bradi and the extensors \triceps and \man including their distinctive fibre orientations from Visible Human Data. The optimization goal is to balance the muscle fibre pre-stretches by activating the extensors and then the flexors alternately with respect to the extension and flexion. The experimental RoM data of the elbow joint were taken from the relevant literature of a healthy person. The muscles’ stress-strain behavior of the pre-stretch optimized design shows a heterogeneous distribution, emphasizing that the structural behavior of the muscle is a complex 3D continuum problem. In addition, the optimal design are compared with handpicked design points with different muscle pre-stretch situations that also fulfill the optimization criterion. The deviations between them are large, \eg, up to 15° in motion and up to 40\,$\%$ muscle force, which underlines the importance of accurate determination of muscles’ fibre pre-stretch.