Design and analysis of a finite element model of the biceps brachii integrated with dynamic in vivo strain fields

The measurement of in vivo behavior of skeletal muscle is crucial to the understanding of muscle function. Non-homogeneity of muscle architecture and surrounding anatomical structures leads to non-uniformity in muscle deformation. Recent techniques in Cine Phase Contrast Magnetic Resonance Imaging (CPC-MRI) has allowed for the non-invasive exploration of internal skeletal muscle deformation. Our group has previously developed novel processing techniques to generate sub-pixel deformation and strain fields of skeletal muscle from CPC-MRI imaging.;This study describes the analysis of strain fields within the biceps brachii generated from dynamic in vivo CPC-MRI data, and the integration of these fields to create, drive, and analyze a finite element model of the biceps brachii. The finite element model allows for the description of the stress fields within the muscle during concentric, isometric, and eccentric contractions at a scale between the sarcomere and overall muscle levels. The model was created to individually represent the intracellular (muscle fiber) and extracellular (matrix) constituents of muscle.;Initially, the strain distributions within the biceps were analyzed during overall concentric, isometric, and eccentric contractions. Non-uniform muscle behavior was observed, with portions of the muscle acting concentrically, isometrically, and eccentrically during all three overall muscle contraction types. Percent area frequency distributions were developed to describe the behavior, with significant differences seen between the contraction types. The strain data produced through this work was then used to drive the muscle model.;The first analysis of the model assessed the influence of muscle properties on the muscle model, due the novel scale of the model. Force-velocity behavior generated significant differences between the stress distributions of the three contraction types for the intracellular elements. Stress distributions were Gaussian shaped, with the average and standard deviation varying between contraction types. The Gaussian distribution of stress within the muscle suggests the presence of protective and energetic strategies employed by the muscle. These strategies are more prevalent during the higher risk eccentric contraction. Eccentric contraction showed the highest average stress, at -100 kPa, and the highest standard deviation of stress, at 31.8 kPa. Posterior regions of the biceps brachii exhibited higher average and deviations of stress compared to anterior regions. The extracellular matrix showed evidence of energy storage and release, especially in the Poisson's effect direction.;A linear relationship between the average stress and activation level in the muscle was generated. As activation increased, the average and standard deviation of stress within the muscle increased. Eccentric contraction experienced the greatest change with a linear coefficient of -1.10914 for average stress and 0.31767 for standard deviation. These relationships provide a method of predicting stress distributions from activation in other muscles with similar morphologies and kinetics.;Stress vs. strain plots were analyzed for the three contraction types to explore the mechanisms the muscle uses to moderate stress during different contractions. Force-length, internal area reorganization, and force-velocity were found to be the most prominent mechanisms in concentric, isometric, and eccentric contractions, respectively.;Possible protective strategies in the muscle were highlighted by the stress distributions, with the muscle maintaining a range of stress to reduce the risk of simultaneous failure of the entire muscle. Energetically, selectively allowing only a portion of the muscle to reach the highest stress levels may reduce the metabolic cost of performing a task. Future work with this study includes the addition of activation measures such as electromyography and the use of optimization techniques to predict activation patterns or material properties of muscle.