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I had a question about actuators and their uses that I am a little unclear about.

I have a model where there are no actuators specified and the only forces present are the various muscle forces defined.

I would like to apply an external force on various joints in the model (I have the information about the force at different instances in time during the trial).

Do I need to specify an actuator inorder to implement the forces or is it sufficient to just select the forces and apply them on the respective joints via the Opensim GUI?

Hi Gregory,

Can you please explain what you mean by "selecting the forces and applying them via the GUI"? Also what's the bigger picture of what you're trying to do?

In the context of a specific tool where you're applying ground reaction forces for example (as we do for Inverse Dynamics) the forces are not added permanently to the model and can be created based on data files. There're other cases where we add actuators during the analysis to act along specific coordinates (e.g. *_RRA_Actuators.xml in gait2354 example).

Hope this helps and please let me know if you have any questions,
-Ayman

In my experimental work I applied a torque at the shoulder and elbow joint of a subject. The actual torque value at every instant is stored in my data file.

I wanted to know whether applying the recorded forces on the respective joints of the upper extremity model is sufficient or whether I have to create an actuator to do it.

The reason I am uncertain about the use is because the arm26 model that is one of the examples provided by Opensim, includes a coordinate actuator, a point actuator and a torque actuator. So I wanted to know whether these needed to be included in a model when you are applying an external force.

Hi Gregory,

The Arm26 does not contain instances of these actuators (it only contains default objects if you're looking at the osim file for book-keeping purposes but these are not part of the model), sorry about the confusion.

-Ayman