Unit of restlength when adding passive spring

[John Chan]

As described in https://simtk-confluence.stanford.edu:8 ... pathspring, these are the information needed when adding a passive spring to the model.

name = 'BiarticularSpringDamper'
restLength = 0.4
stiffness = 10000.0
dissipation = 0.01
pathSpring = modeling.PathSpring(name,restLength,stiffness,dissipation)

My question: What is the unit of the restlength? Is it in metre? How do we decide the restlength of the passive spring?

[Thomas Uchida]

Units are provided in the descriptions of these properties in the API documentation: https://simtk.org/api_docs/opensim/api_ ... pring.html (the "resting_length" parameter is in units of meters, stiffness is in N/m, etc.). The rest length is something the designer would need to determine---it depends on how you want your device to function. For example, one might estimate the rest length based on the lengths of nearby muscle paths, or one might use an optimizer to find the rest length that maximizes some measure of performance for a given activity.

[John Chan]

Thank you for your reply. Is it possible that the restlength to be larger than the optical muscle fibre length?