[MUSCLE] Add support multiple muscle actuator models

[ShravanTata]

The feature, motivation and pitch

Problem

We currently do not have a well-defined way to describe different muscle models needed for neuromechanical simulations. Our implementation mixes three distinct components without clear separation:

1. Transmission (length computation)

   • Joint-based: Muscle length computed as function of joint angles (constant moment arms)
   • MTU-based: Muscle length computed from spatially-defined muscle path (variable moment arms)

2. Activation Dynamics (control → activation)

   • Direct control (pass-through)
   • First-order activation/deactivation dynamics

3. Force Generation (activation → force)

   • Linear combination of control inputs
   • Nonlinear Hill-type force-length-velocity curves (active + passive components)

Proposed Solution

Following MuJoCo's convention, structure muscle models as composable single-input-single-output (SISO) components that can be chained together:

Control Input → Activation Dynamics → Force Generation → Transmission → Joint Torque

Alternatives

No response

FARMS packages

farms_mujoco

Additional context

Muscle models

1. Ekeberg muscles
   • 10.1152/jn.00065.2005
   • https://doi.org/10.1007/BF00199436
2. Brown muscle
   • http://link.springer.com/10.1007/BF00124244
3. Rigid tendon
   • https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3705831/
4. Flexible tendon
   • https://linkinghub.elsevier.com/retrieve/pii/S0021929023002269

Related

• MuJoCo actuator documentation: https://mujoco.readthedocs.io/en/stable/modeling.html#actuator