Starting time in CMC results

[Ratna Sambhav]

Hi all,

I do have a doubt regarding the timing in CMC results. When I see the inversedynamics.sto file of arm26 model, it starts at the time of 0 and steps 0.0083333 seconds forward in time.
When I do static optimization, the time steps as well as start and end time exactly matches with that in inversekinematics.sto results file.
But when I do CMC, time becomes haphazard. It starts with 0.03 seconds, and steps forward by a very small and random time step. Even if I set the start and end time of CMC (using CMC tool) from 0 to 1, it still starts from 0.03 and ends at 0.9.
So, Can someone explain why this keep happening. Is it some kind of error that I am not able to change it?

Thanks

[Carmichael Ong]

Since CMC adds the dynamics of muscles (in comparison with static optimization), the first bit of time is for initializing the muscles.

From: https://simtk-confluence.stanford.edu:8 ... +CMC+Works

Before starting the CMC algorithm, initial states for the model are computed. The states comprise the generalized coordinates (joint angles), generalized speeds (joint angular velocities), plus any muscle states (e.g., muscle activation levels and fiber lengths). While the initial values of the generalized coordinates and speeds can be taken from the desired kinematics that you specify, the initial values of the muscle states are generally unknown. To compute viable starting muscle states, CMC is applied to the first 0.030 seconds of the desired movement. Because the muscle states are generally out of equilibrium and muscle forces can change dramatically during this initial time interval, the simulation results during this interval are generally not valid. Therefore, you should be sure to start CMC at least 0.030 seconds prior to the interval of interest.

As stated above, the 0.03 seconds should not be random. The time steps afterwards SHOULD be random, since the default integrator is a variable time step integrator (rather than a fixed time step integrator).