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The purpose of residual reduction is to minimize the effects of modeling and marker data processing errors that aggregate and lead to large nonphysical compensatory forces called residuals. Specifically, residual reduction alters the torso mass center of a subject-specific model and permits the kinematics of the model from Inverse Kinematics to vary in order to be more dynamically consistent with the ground reaction force data.

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Inputs and Outputs for performing residual reduction. Experimental data are shown in green; OpenSim files (.osim) are shown in red; settings files are shown in blue; files generated by the workflow are shown in purple.

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The subject01_Setup_RRA.xml file is a setup file for the RRA Tool, which specifies settings, inputs, and outputs that affect the behavior of the residual reduction algorithm, which can be defined using the GUI or by hand. Details of the settings are described in the section on using the __40__Graphical User Interface.

The setup file identifies the actuators (i.e., the ideal residual and reserve joint actuators required by RRA) as well as the kinematic tracking tasks. Furthermore, control constraints on the actuators (to limit the maximum residual force) can be specified.

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1. Selection of kp and kv are not arbitrary; they define the behavior of the error dynamics for each q as a second-order linear system. We can write the kp and kv for the desired system behavior in terms of system poles. For a (stable) critically-damped system (real negative poles), kp = λ² and kv = -2λ.
2. This enables kinematics of joints (coordinates) for which we have high confidence (e.g., knee flexion, hip flexion) to be weighted more heavily compared to those of less confidence (e.g., hip internal rotation and ankle inversion).
3. Note that the maximum/minimum force or torque generated by an ideal actuator is the product of the maximum/minimum force and maximum/minimum excitation.
4. Joint torques (and muscles) have a maximum magnitude of 1.
5. Residuals have bounds exceeding their anticipated force requirement. Weightings are implicit in this description. A high optimal_force means that a large output force (torque) does not require a large control value (i.e., low cost). Conversely, residuals with low optimal force require high control values that incur higher costs.

subject01_walk1_grf.xml: ExternalLoads file specifying the measured ground reaction forces that should be applied to the model during simulation and how to apply them.

subject01_simbody.osim: A subject-specific OpenSim model generated by scaling a generic model with the Scale Tool or by other means, along with an associated marker set containing adjusted virtual markers. The model must include inertial parameters.

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