Hyfydy vs MuJoCo

Recently, the MuJoCo simulation engine has been gaining traction for use in biomechanics research. In order to bring awareness of the potential caveats, we highlight the key differences between Hyfydy and MuJoCo.

Hyfydy vs MuJoCo (Website).md

Differences between Hyfydy and MuJoCo

  • The muscle models in Hyfydy simulate tendon elasticity, which is an important mechanism for energy storage-and-release (among many other things) in biomechanical systems[1][2]. MuJoCo does not model this phenomenon.
  • The contact models in Hyfydy include non-linear damping[3] and support separate coefficients for dynamic and viscous friction. MuJoCo contacts do not model these properties.
  • Hyfydy uses error-controlled integration, which adapts the integration step size to ensure robustness and consistent simulation accuracy. MuJoCo uses fixed step size integration without error control.

Differences between Hyfydy, MuJoCo and OpenSim

  • Hyfydy uses the same muscle and contact models as OpenSim, which are well-established in biomechanics research.
  • Hyfydy and MuJoCo are similar in speed, while both are orders of magnitude faster than OpenSim.
  • Hyfydy and MuJoCo both support collision detection and response between a wide range of collision primitives, compared to limited collision detection support in OpenSim.
  • MuJoCo and OpenSim are both free and open source, while Hyfydy is proprietary software.

Feature Comparison Chart

Feature Hyfydy MuJoCo OpenSim
Musculotendon dynamics + - +
Contact Models + +/- +
Collision Detection + + +/-
Accuracy / Error Control + - +
Simulation Speed + + -
Price / Open Source - + +


  1. Blazevich, A. J., & Fletcher, J. R. (2023). More than energy cost: multiple benefits of the long Achilles tendon in human walking and running. Biological Reviews. https://doi.org/https://doi.org/10.1111/brv.13002
  2. Schumacher, P., Geijtenbeek, T., Caggiano, V., Kumar, V., & Schmitt, S. (2023). Natural and Robust Walking using Reinforcement Learning without Demonstrations in High-Dimensional Musculoskeletal Models. (August). Project page https://doi.org/10.13140/RG.2.2.33187.22569/1
  3. Hunt, K. H., & Crossley, F. R. E. (1975). Coefficient of Restitution Interpreted as Damping in Vibroimpact. Journal of Applied Mechanics, 42(2), 440. https://doi.org/10.1115/1.3423596