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Allison J. Nelson, Patrick T. Hall, Katherine R. Saul and Dustin L. Crouch

Wearable passive (ie, spring powered) shoulder exoskeletons could reduce muscle output during motor tasks to help prevent or treat shoulder musculoskeletal disorders. However, most wearable passive shoulder exoskeletons have been designed and evaluated for static tasks, so it is unclear how they affect muscle output during dynamic tasks. The authors used a musculoskeletal model and Computed Muscle Control optimization to estimate muscle output with and without a wearable passive shoulder exoskeleton during 2 simulated dynamic tasks: abduction and upward reach. To an existing upper extremity musculoskeletal model, the authors added an exoskeleton model with 3-dimensional representations of the exoskeleton components, including a spring, cam wheel, force-transmitting shoulder cable, and wrapping surfaces that permitted the shoulder cable to wrap over the shoulder. The exoskeleton reduced net muscle-generated moments in positive shoulder elevation by 28% and 62% during the abduction and upward reach, respectively. However, muscle outputs (joint moments and muscle effort) were higher with the exoskeleton than without at some points of the movement. Muscle output was higher with the exoskeleton because the exoskeleton moment opposed the muscle-generated moment in some postures. The results of this study highlight the importance of evaluating muscle output for passive exoskeletons designed to support dynamic movements to ensure that the exoskeletons assist, rather than impede, movement.

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Amit Gefen

The extrapolation of biological damage from a biomechanical model requires that a closed-form mathematical damage threshold function (DTF) be included in the model. A DTF typically includes a generic load variable, being the critical load (e.g., pressure, strain, temperature) causing irreversible tissue or cell damage, and a generic time variable, which represents the exposure to the load (e.g., duration, strain rate). Despite the central role that DTFs play in biomechanical studies, there is no coherent literature on how to formulate a DTF, excluding the field of heat-induced damage studies. This technical note describes six mathematical function types (Richards, Boltzmann, Morgan-Mercer-Flodin, Gompertz, Weibull, Bertalanffy) that are suitable for formulating a wide range of DTFs. These functions were adapted from the theory of restricted growth, and were fitted herein to describe biomechanical damage phenomena. Relevant properties of each adapted function type were extracted to allow efficient fitting of its parameters to empirical biomechanical data, and some practical examples are provided.

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EDITORIAL Lessons Learned Walter Herzog 1 04 2020 15 03 2020 36 2 57 58 10.1123/jab.2020-0035 jab.2020-0035 COMPUTATIONAL MODEL Effect of Mechanically Passive, Wearable Shoulder Exoskeletons on Muscle Output During Dynamic Upper Extremity Movements: A Computational Simulation Study Allison J

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Daniel C. McFarland, Alexander G. Brynildsen and Katherine R. Saul

motion capture trial. Computational Simulations We performed computed muscle control (CMC) simulations 20 , 21 —augmented to include glenohumeral stability constraints—of abduction and forward flexion (details on data collection used to inform simulations are presented in following subsections). A total

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Amy R. Lewis, William S.P. Robertson, Elissa J. Phillips, Paul N. Grimshaw and Marc Portus

from the extensive physical training 8 , 9 and highly dynamic and physically straining propulsion motion. 10 This is observed across the upper extremity and back musculature, which are demonstrated as influential through both computational simulation 11 – 16 and electromyography. 17 – 21

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Geoffrey T. Burns, Kenneth M. Kozloff and Ronald F. Zernicke

concept, this movement is extremely complex to model and, prior to any computational simulation, was subject to experimental practice and observation. Until the 1960s, jumpers used a variety of techniques to clear the bar, including the Eastern cut-off, the Western roll, and, most commonly, the straddle