OpenSim Musculoskeletal Biomechanics (.osim)

@modelscript/opensim (languages/opensim) provides native WebAssembly parsing, musculoskeletal kinematic tree and mass/inertia validation, Hill-type muscle-tendon activation and contraction dynamics (Thelen2003Muscle, Millard2012EquilibriumMuscle), DAEBuilder lowering, and Modelica 3.x / ROS URDF projections for OpenSim biomechanical and orthopedic surgical planning models.

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Supported Extensions & Standards

Extension	Format	Description
.osim	OpenSim Document	XML specification of BodySet, JointSet, ForceSet (Hill muscles, ligaments, coordinate actuators), MarkerSet, and WrapObjectSet

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Key Capabilities

1. Multibody Skeletal Kinematics & Wrapping Surfaces

• Parses OpenSimDocument (versions 3.x & 4.x) including rigid segments (Body), anatomical joints (PinJoint, SliderJoint, CustomJoint, WeldJoint, FreeJoint), generalized coordinates (Coordinate with range-of-motion bounds), motion-capture fiducial markers (MarkerSet), and cylindrical/ellipsoidal muscle wrapping surfaces (WrapCylinder, WrapSphere, WrapEllipsoid).

2. Hill-Type Muscle-Tendon Activation & Contraction Solver

• Integrates first-order neural excitation-to-activation dynamics ($u(t)\to a(t)$) coupled with active force-length $f_L({\tilde{l}}^M)$, force-velocity $f_V({\tilde{v}}^M)$, passive parallel elastic $f_{PE}({\tilde{l}}^M)$, and pennation angle $\alpha$ tendon equilibrium:$$\frac{da}{dt}=\frac{u-a}{\tau (a,u)},F^T=F_0^M(a{f}_L({\tilde{l}}^M)f_V({\tilde{v}}^M)+f_{PE}({\tilde{l}}^M))\cos \alpha$$

3. DAE Lowering & Modelica / URDF Projection

• Lowers skeletal equations of motion and muscle-tendon ODEs into @modelscript/runtime's DAEBuilder arena and projects models to Modelica.Mechanics.MultiBody (openSimToModelica) and ROS 2 URDF (openSimToUrdf).

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Pre-Flight Diagnostics

10 rules9 error1 warning

Code	Rule	Severity	Message
M9101	opensim-unresolved-body-frame	error	<componentKind> '<componentName>' references undefined body frame '<frameName>'.
M9102	opensim-non-positive-body-mass	error	Body '<bodyName>' specifies invalid mass or principal inertia (<detail>).
M9103	opensim-kinematic-tree-cycle	error	Kinematic tree topology error at joint '<jointName>': <detail>.
M9104	opensim-invalid-coordinate-range	error	Coordinate '<coordName>' has invalid range ['<rangeMin>', '<rangeMax>'] or default value '<defaultValue>' outside range.
M9105	opensim-invalid-muscle-parameters	error	Muscle '<muscleName>' has non-positive Hill parameter (<detail>).
M9106	opensim-excessive-pennation-angle	error	Muscle '<muscleName>' has invalid pennation angle '<angleRad>' rad (must be in [0, pi/2)).
M9107	opensim-insufficient-muscle-path-points	error	Muscle '<muscleName>' has invalid geometry path (<detail>).
M9108	opensim-unresolved-wrap-object	error	Muscle '<muscleName>' references undefined WrapObject '<wrapName>'.
M9109	opensim-invalid-time-constants	warning	Muscle '<muscleName>' has invalid activation/deactivation time constants (tau_act='<tauAct>', tau_deact='<tauDeact>').
M9110	opensim-duplicate-component-name	error	Duplicate <componentKind> name '<name>' declared in OpenSim model.

Generated from languages/opensim/src/lints/rules.ts. All languages →

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CLI Usage

# Inspect bodies, joints, muscles, and markers in an OpenSim model
msx opensim inspect Arm26.osim

# Validate mass/inertia positivity, kinematic cycles, and Hill muscle parameters
msx opensim validate Arm26.osim

# Simulate Hill-type muscle activation and joint trajectories
msx opensim simulate Arm26.osim

# Project OpenSim model to Modelica 3.x or ROS URDF
msx opensim to-modelica Arm26.osim > Arm26.mo
msx opensim to-urdf Arm26.osim > Arm26.urdf