The email answer here for reference, in case others run into this question:
There are actually two sources of stiffness and damping in such a model.
1. The mechanical joint -- In the picture you attached, this would represent the physical stiffness and damping of the robot's mechanical assembly. For example, if there is friction in the joint or you place a spring to make the articulation compliant, this is part of your robot's design.
2. The actuator -- In addition to the joint, the motor has its own dynamic characteristics (resistance, inductivity, inertia, damping, etc.). Typically, motors do not have any springs in them so stiffness is zero. For the damping, you can set this by looking at datasheets or collecting experimental data and estimating the parameter.
With datasheet, you can use parameters like the stall torque and no-load speed to estimate damping from the torque-speed curve.
Using experimental data, you can also estimate parameters