simtk.org

Hello!

I am currently running static optimization for a sit to stand trial. However, I am getting the 1.#QNAN error for a couple frames (one seen below).

time = 0.346665 Performance =8.49846 Constraint violation = 2.85467e-013
time = 0.353332 Performance =8.54855 Constraint violation = 2.65058e-013
time = 0.359998 Performance =8.60453 Constraint violation = 2.76463e-013
time = 0.366665 Performance =0.00939998 Constraint violation = 1.#QNAN
time = 0.373332 Performance =8.76364 Constraint violation = 2.18879e-013
time = 0.379998 Performance =8.82455 Constraint violation = 2.55127e-013

I have looked at past forums and have seen that this error may be caused by noisy data or a sudden jump in accelerations in one frame. My data is filtered (6 Hz). I have also plotted the Inverse Dynamics solution in OpenSim and I do not see any jumps at the time points where I am getting the 1.#QNAN error. Is there anything else I should examine to see what is giving me this error? Thanks!

Elena

The constraint on the static optimization solution requires that the acceleration generated by the model's actuators matches the one estimated from experimental kinematics. A sudden jump in the experimental values may cause the optimizer to fail to meet the constraint tolerance but should not return a NaN. Something is returning a NaN while evaluating the model acceleration or the experimental (e.g. differentiating the spline). A couple things to check: 1) is the analysis starting at a time where you have valid motion data? 2) are kinematics for all coordinates supplied? 3) does the pose of the model make sense- e.g. do any muscles have fiber lengths approaching zero or << optimal_length? If that doesn't uncover a problem, please post your model and setup files so we can take a closer look. Thank you.