Abstract-Dynamic simulations of movement allow one to study neuromuscular coordination, analyze athletic performance, and estimate internal loading of the musculoskeletal system. Simulations can also be used to identify the sources of pathological movement and establish a scientific basis for treatment planning. We have developed a freely available, open-source software system (OpenSim) that lets users develop models of musculoskeletal structures and create dynamic simulations of a wide variety of movements. We are using this system to simulate the dynamics of individuals with pathological gait and to explore the biomechanical effects of treatments. OpenSim provides a platform on which the biomechanics community can build a library of simulations that can be exchanged, tested, analyzed, and improved through a multi-institutional collaboration. Developing software that enables a concerted effort from many investigators poses technical and sociological challenges. Meeting those challenges will accelerate the discovery of principles that govern movement control and improve treatments for individuals with movement pathologies.Index Terms-Computed muscle control, forward dynamic simulation, musculoskeletal modeling, open-source software.
A fundamental question in movement science is how humans perform stable movements in the presence of disturbances such as contact with objects. It remains unclear how the nervous system, with delayed responses to disturbances, maintains stability of complex movements. We hypothesized that intrinsic muscle properties (i.e., the force-length-velocity properties of muscle fibers and tendon elasticity) may help stabilize human walking by responding instantaneously to a disturbance and providing forces that help maintain the movement trajectory. To investigate this issue we generated a three-dimensional muscle-driven simulation of walking and analyzed changes in the simulation’s motion when a disturbance was applied to models with and without intrinsic muscle properties. Removing intrinsic properties reduced stability; this was true when the disturbing force was applied at a variety of times and in different directions. Thus, intrinsic muscle properties play a unique role in stabilizing walking, complementing the delayed response of the central nervous system.
Impaired control of mediolateral body motion during walking is an important health concern. Developing treatments to improve mediolateral control is challenging, partly because the mechanisms by which muscles modulate mediolateral ground reaction force (and thereby modulate mediolateral acceleration of the body mass center) during unimpaired walking are poorly understood. To investigate this, we examined mediolateral ground reaction forces in eight unimpaired subjects walking at four speeds and determined the contributions of muscles, gravity, and velocity-related forces to the mediolateral ground reaction force by analyzing muscle-driven simulations of these subjects. During early stance (0-6% gait cycle), peak ground reaction force on the leading foot was directed laterally and increased significantly (p < 0.05) with walking speed. During early single support (14-30% gait cycle), peak ground reaction force on the stance foot was directed medially and increased significantly (p < 0.01) with speed. Muscles accounted for more than 92% of the mediolateral ground reaction force over all walking speeds, whereas gravity and velocity-related forces made relatively small contributions. Muscles coordinate mediolateral acceleration via an interplay between the medial ground reaction force contributed by the abductors and the lateral ground reaction forces contributed by the knee extensors, plantarflexors, and adductors. Our findings show how muscles that contribute to forward progression and body-weight support also modulate mediolateral acceleration of the body mass center while weight is transferred from one leg to another during double support.
Control of lateral balance during walking can be hindered by aging, stroke, or neuromuscular impairments [1, 2, 3]. Lateral balance can be characterized using the medial-lateral acceleration of the mass center of the body, yet the mechanisms by which muscles coordinate medial-lateral acceleration of the body mass center during walking are poorly understood. Elucidation of these mechanisms could help improve rehabilitative or surgical treatments for improving lateral balance in patients with walking impairments. For example, if the muscles that contributed most to medial-lateral acceleration of the body mass center were known, strengthening these muscles could be the focus of therapy. In this study, we analyzed a three-dimensional, muscle-driven simulation of unimpaired walking to quantify the contributions of individual muscles to the medial-lateral acceleration of the body mass center.
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