Confusing with force value by JointReaction Analyze

[Jianwei Qiu]

Hi everyone,
I am using Tools -> Analyze (OpenSim 3.1) to calculate Gait_2354 model joint reaction.

In the 'Main Settings' pane: 'Controls' and 'Solve for equilibrium for actuator states' are not checked, and file 'subject01_walk1_ik.mot' is used as motion.
In the 'Actuators and External Loads' pane: no 'Additional force set files' is selected, and file 'subject01_walk1_grf.xml' is used as 'External Loads'.
In the 'Analyses' pane: 'JointReaction' is added.

Analyze results: S.I. units are adopted in the results, at time 0, values of hip_r_on_femur_r_in_ground_fx, hip_r_on_femur_r_in_ground_fy, hip_r_on_femur_r_in_ground_fz are -42838.41967167 N, -85536.15999402 N, -5282.51892978 N, respectively.

My question is: the weight of Gait_2354 model is about 75kg, how could these three values be so large ?

Please help me, thank you.
Qiu

[jimmy d]

Hi Qiu,

those results suggest that something is wrong with the simulation and/or data input. Often when there are errors of large orders of magnitude its because the experimental data units are incorrect.

Let us know if that helps,
-james

[Paige Kendell]

Hi Qiu,

Did you ever figure out what was causing such large force values? I'm having a similar issue, only my reported force values are too low (looking at the knee joint). I believe my experimental data units are correct.

I have some differences in my setup file. I use states results from CMC and do not solve for equilibrium actuator states. I replace the model's force set with CMC_actuators.xml and then use ground reaction force data for the external loads. And then obviously, I have the JointReaction analysis selected in the Analysis pane.

Thanks,
Paige