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How can I obtain the residual forces at the knee (I can get the moments) - do I have to set up a coordinate in the osim file? The knee has two dependent coordinates - x and y. Can I retrieve the residual for these coorrdinates after RRA and CMC? Will have to run RRA and CMC again or is there a way to extract the force residuals without a rerun?

Thanks
Julia

Residual forces are required because of inconsistencies between experimental forces, measured kinematics and the model, and is therefore a measure of error. We usually apply residual actuator to the pelvis to determine how much residual force must be applied to track the desired kinematics. Are you applying residual actuators to the knee? You can think of RRA as being similar to inverse dynamics (yields the generalized forces corresponding to the generalized coordinates) but kinematics are allowed to deviate and mass adjustments made to reduce the residual (error) forces. In this context, what do you mean by "residual forces at the knee?"

If I am not wrong then residual forces and moments can be applied at the pelvis - in that case, can both residual forces and moments be applied at the knee? In case of RRA, actuator moment or torque is reported for the knee(Nm). Can we obtain a similar force at the knee in RRA?

Hi Julia,

You *could* assign residual forces to one of the segments about the knee, or perhaps at the knee joint, but that's not really what the residual forces are for. You can think of them as a general "What forces need to be added to keep the model dynamically consistent." For instance, if you were running a model that only consisted of the lower body but applied ground reaction forces representing a movement done by someone with a whole body, the residual forces would be quite large as the model would go flying away if they weren't applied. They're typically applied at the pelvis because it's a fairly convenient place to do so in terms of how the model is constructed; applying them at the knee may get a little messy if I'm not mistaken.

Are you looking for a measurement of error in the forces that the actuators are applying at the knee? Something like "The quadriceps applied 30N more than they needed to" or something of that nature?

danielleib wrote:Hi Julia,

You *could* assign residual forces to one of the segments about the knee, or perhaps at the knee joint, but that's not really what the residual forces are for. You can think of them as a general "What forces need to be added to keep the model dynamically consistent." For instance, if you were running a model that only consisted of the lower body but applied ground reaction forces representing a movement done by someone with a whole body, the residual forces would be quite large as the model would go flying away if they weren't applied. They're typically applied at the pelvis because it's a fairly convenient place to do so in terms of how the model is constructed; applying them at the knee may get a little messy if I'm not mistaken.

Are you looking for a measurement of error in the forces that the actuators are applying at the knee? Something like "The quadriceps applied 30N more than they needed to" or something of that nature?

Dear Daniel:
If I am not mistaken, can we regard the calculated residual actuator as some external forces that is unknown but essential for dynamic consistence ?
Too many cases need this, hence in my opinion it is quite useful in this way.

Regards
Jack