How muscle stiffness affects human body model behavior

Abstract

Background Active human body models (AHBM) consider musculoskeletal movement and joint stiffness via active muscle truss elements in the finite element (FE) codes in dynamic application. In the latest models, such as THUMS™ Version 5, nearly all human muscle groups are modeled in form of one-dimensional truss elements connecting each joint. While a lot of work has been done to improve the active and passive behavior of this 1D muscle system in the past, the volumetric muscle system of THUMS was modeled in a much more simplified way based on Post Mortem Human Subject (PMHS) test data. The stiffness changing effect of isometric contraction was hardly considered for the volumetric muscle system of whole human body models so far. While previous works considered this aspect for single muscles, the effect of a change in stiffness due to isometric contraction of volumetric muscles on the AHBM behavior and computation time is yet unknown. Methods In this study, a simplified frontal impact using the THUMS Version 5 AM50 occupant model was simulated. Key parameters to regulate muscle tissue stiffness of solid elements in THUMS were identified for the material model MAT_SIMPLIFIED_FOAM and different stiffness states were predefined for the buttock and thigh. Results During frontal crash, changes in muscle stiffness had an effect on the overall AHBM behavior including expected injury outcome. Changes in muscle stiffness for the thigh and pelvis, as well as for the entire human body model and for strain-rate-dependent stiffness definitions based on literature data had no significant effect on the computation time. Discussion Kinematics, peak impact force and stiffness changes were in general compliance with the literature data. However, different experimental setups had to be considered for comparison, as this topic has not been fully investigated experimentally in automotive applications in the past. Therefore, this study has limitations regarding validation of the frontal impact results. Conclusion Variations of default THUMS material model parameters allow an efficient change in stiffness of volumetric muscles for whole AHBM applications. The computation time is unaffected by altering muscle stiffness using the method suggested in this work. Due to a lack of validation data, the results of this work can only be validated with certain limitations. In future works, the default material models of THUMS could be replaced with recently published models to achieve a possibly more biofidelic muscle behavior, which would even allow a functional dependency of the 1D and 3D muscle systems. However, the effect on calculation time and model stability of these models is yet unknown and should be considered in future studies for efficient AHBM applications.

Scapular muscle dynamic stiffness of asymptomatic subjects and subjects with chronic shoulder pain, at rest and isometric contraction conditions

“…118 Thus, these models that have been improved over the years to be closer to the human reality, 120,121 can actually reflect the muscle stiffness 120 with agreement with the data obtained by elastography. 120 Therefore, this kind of approaches can be used to relate stiffness variations with the risk of injury 122 or to distinguish subjects with and without pain conditions. 54,55 Moreover, the assessment of subjects scapular position, as proposed by some studies, 123–125 would be relevant given its possible relation with muscular mechanical properties through variations of muscular function or length.…”

“…46 The mentioned problem could also be solved, if it would be possible to considered each participant specific characteristics, by simulation studies (possibly considering also the existent myotonometry data) once there are already models created considering scapular kinematics 117,118 and muscular geometry and behavior. 54,[118][119][120] More specifically, previous studies already built some models for shoulder complex (bones, joints, ligaments, and muscles) 118,119 and, try to built models considering properties as muscle texture properties as muscle texture/deformation. 118 Thus, these models that have been improved over the years to be closer to the human reality, 120,121 can actually reflect the muscle stiffness 120 with agreement with the data obtained by elastography.…”

“…54,[118][119][120] More specifically, previous studies already built some models for shoulder complex (bones, joints, ligaments, and muscles) 118,119 and, try to built models considering properties as muscle texture properties as muscle texture/deformation. 118 Thus, these models that have been improved over the years to be closer to the human reality, 120,121 can actually reflect the muscle stiffness 120 with agreement with the data obtained by elastography. 120 Therefore, this kind of approaches can be used to relate stiffness variations with the risk of injury 122 or to distinguish subjects with and without pain conditions.…”