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Abstract ObjectivesMusculoskeletal modeling is a powerful approach for studying the biomechanics and energetics of locomotion.Australopithecus (A.) afarensisis among the best represented fossil hominins and provides critical information about the evolution of musculoskeletal design and locomotion in the hominin lineage. Here, we develop and evaluate a three‐dimensional (3‐D) musculoskeletal model of the pelvis and lower limb ofA. afarensisfor predicting muscle‐tendon moment arms and moment‐generating capacities across lower limb joint positions encompassing a range of locomotor behaviors. Materials and MethodsA 3‐D musculoskeletal model of an adultA. afarensispelvis and lower limb was developed based primarily on the A.L. 288‐1 partial skeleton. The model includes geometric representations of bones, joints and 35 muscle‐tendon units represented using 43 Hill‐type muscle models. Two muscle parameter datasets were created from human and chimpanzee sources. 3‐D muscle‐tendon moment arms and isometric joint moments were predicted over a wide range of joint positions. ResultsPredicted muscle‐tendon moment arms generally agreed with skeletal metrics, and corresponded with human and chimpanzee models. Human and chimpanzee‐based muscle parameterizations were similar, with some differences in maximum isometric force‐producing capabilities. The model is amenable to size scaling from A.L. 288‐1 to the larger KSD‐VP‐1/1, which subsumes a wide range of size variation inA. afarensis. DiscussionThis model represents an important tool for studying the integrated function of the neuromusculoskeletal systems inA. afarensis. It is similar to current human and chimpanzee models in musculoskeletal detail, and will permit direct, comparative 3‐D simulation studies. 

MocoExtendProblem (MEP) is a framework to rapidly develop novel goals for biomechanical optimal control problems using OpenSim Moco (Dembia et al., 2020) and MATLAB (The MathWorks, Inc., Natick, MA, USA). MEP features several templates for testing and prototyping novel MocoGoals as well as a build tool to create a MEX function using OpenSim’s API for MATLAB in lieu of rebuilding OpenSim from source or building a plugin and generating an .omoco file from C++ to load the problem into MATLAB. Instead, users structure and design custom goals in C++, build them with the provided tool, and call custom goals from within MATLAB scripts. This repository features: • A build.m script that compiles goals in the custom_goals directory and procedurally constructs the C++/MATLAB class implementations and compiles the MEX interface. • Compatibility tested with OpenSim 4.2-4.5. – Support for OpenSim versions 4.2-4.4 require unique considerations to custom goal development and build pipeline since Booleans for division by duration, distance and mass were migrated to the abstract MocoGoal. • The ability to include MEP as a submodule, build, and use valid custom goals. • Three example custom goals in the custom_goals and custom_goals_compat directories. 

Herein, the dataset generated for Queeno et al. [1] is presented and described. Mammalian skeletal muscle slow (MyHC-I) fiber composition data was collated from 269 eligible studies identified via a systematic literature search and meta-analysis, following a structure similar to PRISMA [2]. Academic search systems were queried with terms relating to mammalian skeletal muscle fiber content and reference lists of selected articles were thoroughly investigated for additional studies. Eligible studies were those that provided skeletal muscle fiber composition data from mammalian species that were not subjected to experimental manipulations. Taxonomic information, sex, age, number of individuals sampled, average body mass (kg), average slow fiber content (%) of each skeletal muscle under investigation and fiber-typing methodology were collated from eligible studies when available. Muscle fiber composition data was collected from more than 200 skeletal muscles across 174 mammalian species, which will be of value to those interested in muscle physiology, interspecific muscle comparisons, and connections between muscle physiology, taxonomy, body mass, ecomorphology and locomotor strategy (among others). 

Humans are unique among terrestrial mammals in our manner of walking and running, reflecting 7 to 8 Ma of musculoskeletal evolution since diverging with the genus Pan. One component of this is a shift in our skeletal muscle biology towards a predominance of myosin heavy chain (MyHC) I isoforms (i.e. slow fibers) across our pelvis and lower limbs, which distinguishes us from chimpanzees. Here, new MyHC data from 35 pelvis and hind limb muscles of a Western gorilla (Gorilla gorilla) are presented. These data are combined with a similar chimpanzee dataset to assess the MyHC I content of humans in comparison to African apes (chimpanzees and gorillas) and other terrestrial mammals. The responsiveness of human skeletal muscle to behavioral interventions is also compared to the human-African ape differential. Humans are distinct from African apes and among a small group of terrestrial mammals whose pelvis and lower limb muscle is slow fiber dominant, on average. Behavioral interventions, including immobilization, bed rest, spaceflight and exercise, can induce modest decreases and increases in human MyHC I content (i.e. -9.3% to 2.3%, n = 2033 subjects), but these shifts are much smaller than the mean human-African ape differential (i.e. 31%). Taken together, these results indicate muscle fiber content is likely an evolvable trait under selection in the hominin lineage. As such, we highlight potential targets of selection in the genome (e.g. regions that regulate MyHC content) that may play an important role in hominin skeletal muscle evolution.