Hello, I am working to implement the muscle spindle model described in this paper. The parameters that it needs are FM : The force exerted by the muscle fiber FM (N) FMmax : Maximal isometric force (N) lM : Fiber length (mm) lMopt : Optimal fiber length (mm) == rest sarcomere length a : Normalized muscle activity vM : Fiber contraction velocity (mm/s) I'm working on getting these values out of Mujoco. So far I'm looking at FM : sim.data.actuator_force FMmax : the user specified 'force' if specified, otherwise scale / actuator_acc0 lM : Where I'm looking for the length of the muscle, and actuator_length is the muscle length plus the length of the tendon sim.data.actuator_length - sim.model.tendon_length0 but I'm not sure that tendon_length0 is actually what I'm looking for. I might be looking for sim.data.ten_length? It's not clear to me what the difference is. The documentation says that tendon_length0 is the tendon length in qpos0, but also the length of the tendons is supposed to be constant, if I understood the documentation properly. lMopt: Working backwards from the equations provided in the documentation, I'm calculating the resting muscle length (L0 in the Mujoco documentation) for muscle ii as: (lengthrange[ii][0] - lengthrange[ii][1]) / (gainprm[ii][0] - gainprm[ii][1]) a I'm not sure here what value to use. I see that the actuator_force = - FLV(L, V, act) * F0 so I would guess a = actuator_force / F0 but I'd like to be sure I'm not mixing up terms or missing something. vM: The fiber contraction velocity I'm guessing in this case is sim.data.actuator_velocity ... ? There's also ten_velocity, and it's not clear to me how they interact, exactly. Any insight that you could provide would be greatly appreciated! Thanks,