Difference between Inverse Dynamics torque and forward dynamics torque

[Shivangi Giri]

Hi,
I am calculating Inverse Dynamics torque at the elbow using the Opensim ID solver and then using this torque to calculate the muscle activations in arm26 model using a python script. I then calculate the FD torque (summing up both active and passive muscles torque), however the torques from ID and FD don't match.
I have implemented PD controller for trajectory mapping in the ID with kp=400, kv=40. I understand this would initially cause a significant deviation between the ID and FD, however after the few initial time-steps the torque values from both ID and FD should be close to each other.
Can someone please help in understanding why the difference?
Regards,
Shivangi