_RRA Best Practices

RRA Settings:

1. You should replace the muscles in your model with residual actuators and ideal joint actuators. Residual reduction is a form of forward dynamics simulation that utilizes a tracking controller to follow model kinematics determined from the inverse kinematics. Computed Muscle Control (CMC) serves as the controller, but without muscles, the skeleton of the model can be used to determine a mass distribution and joint kinematics that are more consistent with ground reaction forces.
2. Optimal forces for residuals should be low to prevent the optimizer from "wanting" to use residual actuators (an actuator with large optimal force and low excitation is "cheap" in the optimizer cost).
3. To help minimize residuals, make an initial pass with default inputs, then check residuals and coordinate errors. To reduce residuals further, decrease tracking weights on coordinates with low error. You can also try decreasing the maximum excitation on residuals or the actuator optimal force.
4. Typically, you should "lock" the subtalar and mtp joints in the *.osim file.
5. Make sure "use_fast_optimization_target" is false. This allows the kinematics to be slightly adjusted to account for dynamic inconsistencies. This is the default in the settings files distributed with OpenSim or created from the GUI. See How CMC Works for a comparison of the "slow" and "fast" targets. In the GUI, this option is hidden for RRA and can be viewed by opening the xml settings file.
6. The "cmc_time_window" in the settings file should be 0.001 s for RRA. This is the default in the settings files distributed with OpenSim or created from the GUI.
7. See How RRA Works and How to Use the RRA Tool for more information about RRA settings.

Troubleshooting:

1. Check the pelvis COM location in the Actuator files.
2. If RRA is failing, try increasing the maximum excitation for residuals by orders of magnitude until the simulation runs, then try working your way back down while also "relaxing" tracking weights on coordinates.
3. If residuals are very large (typically, this is greater than 2-3 times body weight, depending on the motion), there is probably something wrong with either (i) the scaled model, (ii) the IK solution, or (iii) the applied GRFs. To double-check that forces are being applied properly, visualize GRFs with IK data (you can use the Previewing Motion Capture (Mocap) Data function in the GUI).
4. If there is pelvis drift and/or F_Y is not centered around zero, check that the body mass and force calibration are correct.
5. When using the example RRA Actuators .xml file, you should note that residual forces are applied to the center of mass (COM) of the unscaled pelvis; however, if you scale the model, the COM of the pelvis can change. Although the effect may be small, you should change the location of the residual force actuators in the RRA Actuators file to correspond to the scaled pelvis COM.

Evaluating your Results:

1. RMS difference in joint angle during the movement should be less than 2-5º (or less than 2 cm for translations).
2. Peak residual forces should typically be less than 10-20 N. Average residuals should typically be less than 5-10 N.
   
   1. The size of residuals will depend on the type of motion being studied. For example, residuals for high-speed activities like sprinting will typically be larger than walking.
   2. Residuals will also be larger if there are external forces that you have not accounted for, such as a subject walking with a handrail.
3. Compare the residual moments from RRA to the moments from Inverse Dynamics. You should see a 30-50% reduction in peak residual moments.
4. Compare the joint torques/forces to established literature (if available). Try to find data with multiple subjects. Your results should be within one standard deviation of the literature.

The table below shows an example of threshold values used to evaluate RRA results for full-body simulations of walking and running.