MuSkeMo: Open-source software to construct, analyze, and visualize human and animal musculoskeletal models and movements in Blender

Musculoskeletal models for multibody dynamic analysis provide unique insights into human and animal movement. Although some biomechanical simulators provide model-building tools, these presuppose substantial preprocessing by the user, and resulting models are generally not cross-platform compatible. Thus, the workflow from anatomical 3D scans to musculoskeletal model is time-consuming, requiring numerous processing and conversions steps between software packages, and the process differs between simulators. Despite the popularity of musculoskeletal modelling within biomechanics, no cross-platform, open source software package exists for constructing musculoskeletal models. Here, I introduce MuSkeMo: A software suite for defining 3D musculoskeletal models entirely within Blender (open-source 3D computer graphics software). MuSkeMo provides a visual interface, enabling users to interactively define all aspects of a musculoskeletal model (including rigid bodies, skeletal geometry, joint centres, muscles, landmarks, and body-fixed reference frames). MuSkeMo can calculate 3D inertial tensors from arbitrary meshes (e.g., from CT scans), and also implements automated convex-hull based mass-estimation approaches from the literature. Joints can be defined using shape-fitting of bony surfaces, and muscles can wrap around primitive shapes. Models can be analyzed within MuSkeMo using popular pose-sampling procedures, or exported to multiple text-based formats for use in biomechanical simulators. A conversion script to OpenSim is included. MuSkeMo is compatible with models created for popular biomechanical simulators (OpenSim and Gaitsym). MuSkeMo can import these models and simulation trajectories, enabling users to create publication-ready stills and animations with Blenders ray tracing. These visualisations include volumetric muscles based on the contractile parameters, which can be more visually intuitive than traditional constant-diameter tube segments. Whether the end goal is a highly-detailed subject-specific human model, or a simplified animal model, MuSkeMo includes features that can aid this process. By consolidating many elements of common model construction workflows into a cohesive package, MuSkeMo substantially simplifies musculoskeletal modelling.