Hyfydy works different than most physics engines, in that it is fully force-driven. Even joint constraints are modeled through forces that mimic the effects of ligaments and cartilage. Instead of solving constraints directly, Hyfydy is optimized to run at high simulation frequencies (>3000Hz), and uses a novel error-controlled integrator that automatically adapts the step size based on a user-defined accuracy level. This results in an approximate 100x speed gain* over OpenSim  — while using the same muscle and contact models. Hyfydy can perform reflex-based gait simulations  in around 150x real-time (Intel i7 CPU, single-core).
Hyfydy contains optimized implementations of various Hill-type musculotendon actuators, including the Millard Equilibrium Muscle Model . This model incorporates tendon elasticity, variable pennation angles and passive damping of the contractile element, allowing muscles to be fully deactivated. Hyfydy also includes several other actuator models, such as rigid tendon muscle actuators and joint torque actuators.
Hyfydy supports a wide range of force-based contact models, including the Hunt-Crossley Contact Model  with non-linear damping. These models better represent soft-tissue contact forces than rigid contact models. The Coulomb friction cone is modeled using static, dynamic and viscous friction coefficients. The error-controlled integrator with adaptive time steps ensures that the simulation stiffness associated with force-based contact models has a minimal impact on performance.
Accuracy & Stability
Hyfydy supports a number of error-controlled integrators with adaptive step sizes and user-controlled accuracy. This ensures that simulations in Hyfydy always remains stable and accurate, while retaining optimal performance.
Hyfydy is continuously being improved and new features are added regularly based on user requests. It is currently available as a plug-in for SCONE , which is Open Source Software for predictive simulations of biological motion, as well as via a Python API for machine learning applications. For specific feature requests, please don’t hesitate to send us a message.
* Performance benchmark details: 10 second gait simulation using a reflex-based controller  with hill-type musculotendon dynamics  and non-linear contact forces . Optimizations were performed in SCONE  using 1000 generations of CMA-ES  on a 4-core i7 Intel CPU.
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