OpenSim vs Step

OpenSim models and simulates how humans, animals, robots, and their environment move and interact. Its graphical interface displays models and supports creating and analyzing simulations. Models can contain rigid bodies, joints, constraints, contact, muscles, and other actuators. Movement analysis tools include inverse kinematics, inverse dynamics, static optimization, and forward dynamics; Computed Muscle Control can generate muscle-driven forward simulations. The API is usable through Matlab, Python, Java, and C++, and GUI scripting uses an embedded Jython shell. Plugins can add analyses and model components, but must be built for the same OpenSim version and platform. The graphical interface is distributed for 64-bit Windows and Mac, while there is no public Linux GUI distribution; the API source can be built for Linux. OpenSim is free and open source under Apache 2.0, though models, examples, and plugins may have separate licenses. It is intended for research and teaching and welcomes academic and commercial applications. Documentation, tutorials, and a discussion forum are available. Downloading requires a SimTK account.

Step is a free Linux desktop application for exploring physical systems through interactive simulations. Users can change body and force properties while a simulation runs. Supported models include two-dimensional classical mechanics, molecular dynamics, sound waves, and deformable bodies, with particles, damped springs, gravitational and Coulomb forces, rigid bodies, and collision handling. Step calculates unit conversions, expressions, error propagation, and solver error estimates. Explicit and implicit solvers reach 8th order, with optional adaptive timesteps; most require GSL. Graph, meter, and tracer tools visualize results, while a controller changes properties using keyboard shortcuts. Tutorials, example experiments, object information, and an integrated Wikipedia browser provide learning material. More examples can be downloaded through KNewStuff. Expression calculations such as unit conversion require libqalculate. Step is GPL-2.0+ software, and its StepCore library can be used separately. The listed release is Step 26.08.2, released 2026-10-08.