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Hi Everyone,

I am trying to compare the ID pelvic residual results from a cycling trial to experimental data measured from transducers at the seat and handlebars. Looking at the ID results, I can't figure out whether the pelvic residuals are presented with respect to the ground coordinate system or to the local pelvis coordinate system. Does anyone know how these forces and moments are reported? I'm using the gait2392 model with OpenSim 3.3.

Thanks and let me know if I need to clarify anything!

Brad

The results form ID are the generalized forces that satisfy the motion, externally and internally applied forces. The value of the generalized force acts in the direction of the degree of freedom and doesn't need to be expressed in some particular frame. It is not a property of the body, but a property of the joint that permits movement in a particular direction.

You may confuse this with the body forces that are applied to the bodies. In that case, you can express the vector of forces/torques into a different frame of interest in order to simplify the calculations. Since Simbody/OpenSim uses generalized coordinates and not absolute coordinates the body forces and generalized forces are different, but they are related through the system body Jacobian (tau = J^T * f_body).

In the case of the pelvis body, things are a little bit different since it is a 6D joint connected to the ground. The body forces and generalized forces coincide and the results are expressed in the local frame of the pelvis (i.g. in the direction of the degrees of freedom).