MUSCLE INERTIA DURING RUNNING: A MASSIVE CHANGE OF MOMENTS?

Skeletal muscles have substantial inertia that cause inertial forces working around joints. These inertial forces are not typically considered in musculoskeletal models used for sport biomechanics research, which can lead to considerable errors in estimated joint kinetics. How large these errors are in common sports movements is yet unclear. We therefore examined the role of shank muscle inertia on ankle joint moments during the swing phase of running at different speeds. Ankle moments were considerably affected when muscles were modelled as separate masses, with a general shift towards reduced dorsiflexion and higher plantarflexion moments. These results show that ignoring inertial muscle forces in musculoskeletal simulations can lead to under- or overestimations of structure-specific loads and possibly erroneous conclusions. We therefore encourage sport biomechanics researchers to consider the impact of muscle inertia on inverse dynamics calculations

Boxelization: folding 3D objects into boxes

We present a method for transforming a 3D object into a cube or a box using a continuous folding sequence. Our method produces a single, connected object that can be physically fabricated and folded from one shape to the other. We segment the object into voxels and search for a voxel-tree that can fold from the input shape to the target shape. This involves three major steps: finding a good voxelization, finding the tree structure that can form the input and target shapes' configurations, and finding a non-intersecting folding sequence. We demonstrate our results on several input 3D objects and also physically fabricate some using a 3D printer

Data-driven finite elements for geometry and material design

Crafting the behavior of a deformable object is difficult---whether it is a biomechanically accurate character model or a new multimaterial 3D printable design. Getting it right requires constant iteration, performed either manually or driven by an automated system. Unfortunately, Previous algorithms for accelerating three-dimensional finite element analysis of elastic objects suffer from expensive precomputation stages that rely on a priori knowledge of the object's geometry and material composition. In this paper we introduce Data-Driven Finite Elements as a solution to this problem. Given a material palette, our method constructs a metamaterial library which is reusable for subsequent simulations, regardless of object geometry and/or material composition. At runtime, we perform fast coarsening of a simulation mesh using a simple table lookup to select the appropriate metamaterial model for the coarsened elements. When the object's material distribution or geometry changes, we do not need to update the metamaterial library---we simply need to update the metamaterial assignments to the coarsened elements. An important advantage of our approach is that it is applicable to non-linear material models. This is important for designing objects that undergo finite deformation (such as those produced by multimaterial 3D printing). Our method yields speed gains of up to two orders of magnitude while maintaining good accuracy. We demonstrate the effectiveness of the method on both virtual and 3D printed examples in order to show its utility as a tool for deformable object design.National Science Foundation (U.S.) (Grant CCF-1138967)United States. Defense Advanced Research Projects Agency (N66001-12-1-4242

Assemble Them All: Physics-Based Planning for Generalizable Assembly by Disassembly

Assembly planning is the core of automating product assembly, maintenance, and recycling for modern industrial manufacturing. Despite its importance and long history of research, planning for mechanical assemblies when given the final assembled state remains a challenging problem. This is due to the complexity of dealing with arbitrary 3D shapes and the highly constrained motion required for real-world assemblies. In this work, we propose a novel method to efficiently plan physically plausible assembly motion and sequences for real-world assemblies. Our method leverages the assembly-by-disassembly principle and physics-based simulation to efficiently explore a reduced search space. To evaluate the generality of our method, we define a large-scale dataset consisting of thousands of physically valid industrial assemblies with a variety of assembly motions required. Our experiments on this new benchmark demonstrate we achieve a state-of-the-art success rate and the highest computational efficiency compared to other baseline algorithms. Our method also generalizes to rotational assemblies (e.g., screws and puzzles) and solves 80-part assemblies within several minutes.Comment: Accepted by SIGGRAPH Asia 2022. Project website: http://assembly.csail.mit.edu

Simit: A Language for Physical Simulation

Using existing programming tools, writing high-performance simulation code is labor intensive and requires sacrificing readability and portability. The alternative is to prototype simulations in a high-level language like Matlab, thereby sacrificing performance. The Matlab programming model naturally describes the behavior of an entire physical system using the language of linear algebra. However, simulations also manipulate individual geometric elements, which are best represented using linked data structures like meshes. Translating between the linked data structures and linear algebra comes at significant cost, both to the programmer and the machine. High-performance implementations avoid the cost by rephrasing the computation in terms of linked or index data structures, leaving the code complicated and monolithic, often increasing its size by an order of magnitude. In this paper, we present Simit, a new language for physical simulations that lets the programmer view the system both as a linked data structure in the form of a hypergraph, and as a set of global vectors, matrices and tensors depending on what is convenient at any given time. Simit provides a novel assembly construct that makes it conceptually easy and computationally efficient to move between the two abstractions. Using the information provided by the assembly construct, the compiler generates efficient in-place computation on the graph. We demonstrate that Simit is easy to use: a Simit program is typically shorter than a Matlab program; that it is high-performance: a Simit program running sequentially on a CPU performs comparably to hand-optimized simulations; and that it is portable: Simit programs can be compiled for GPUs with no change to the program, delivering 5-25x speedups over our optimized CPU code

Strand-based musculotendon simulation of the hand

This dissertation develops a framework for modelling biomechanical systems, with special focus on the muscles, tendons, and bones of the human hand. Two complementary approaches for understanding the functions of the hand are developed: the strand simulator for computer modelling, and an imaging apparatus for acquiring a rich data set from cadaver hands. Previous biomechanical simulation approaches, based on either lines-of-force or solid mechanics models, are not well-suited for the hand, where multiple contact constraints make it difficult to route muscles and tendons effectively. In lines-of-force models, wrapping surfaces are used to approximate the curved paths of tendons and muscles near joints. These surfaces affect only the kinematics, and not the dynamics, of musculotendons. In solid mechanics models, the 3D deformation of muscles can be fully accounted for, but these models are difficult to create and expensive to simulate; moreover, the fibre-like properties of muscles are not directly represented and must be added on as auxiliary functions. Neither of these approaches properly handles both the dynamics of the musculotendons and the complex routing constraints. We present a new, strand-based approach, capable of handling the coupled dynamics of muscles, tendons, and bones through various types of routing constraints. The functions of the hand can also be studied from the analysis of data obtained from a cadaver hand. We present a hardware and software setup for scanning a cadaver hand that is capable of simultaneously obtaining the skeletal trajectory, tendon tension and excursion, and tendon marker motion. We finish with a preliminary qualitative comparison of a simulation model of the index finger with real world data acquired from ex vivo specimen, using the strands framework.Science, Faculty ofComputer Science, Department ofGraduat