Effect of External Loads on Static Optimization/Joint Reaction Analysis

[Ryan Byrne]

Hi everyone,

I am working on a full-body model and am interested in estimating the joint forces and moments during a lifting motion. I am attempting to run Static Optimization and then Joint Reaction Analysis and have had some success but have run into an issue. From what I understand, the gndpelvis forces and moments from the Joint Reaction Analysis should be zero or very close to zero if everything is running smoothly (I have 6 DOF in my model, also). However, in my case the forces and My are zero, while Mx and Mz are not. The Mz value is large - around 300 Nm.

To try and find a solution, I took a step back and removed my external forces (ground reaction forces) to see what the solution was like. The gndpelvis Mz moment reduced to around 50 Nm, but I noticed that the reaction forces at the lumbar joints, and seemingly everything else above the pelvis, were almost exactly the same without the external forces. Does anyone know why the external loads may only be affecting the joints at or below the pelvis, or have any suggestions as to how to troubleshoot?

Thanks,
Ryan