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.

The Residual Reduction Algorithm (RRA) Tool is accessed by selecting Tools → Residual Reduction Algorithm… from the OpenSim main menu bar. Like all tools, the operations performed by the Residual Reduction Algorithm Tool apply to the current model.

Overview

The figure below shows the required inputs and outputs for performing the residual reduction algorithm. Each is described in more detail below.

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.

Settings File

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 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.

Inputs

Several files are required as input to the RRA Tool to perform residual reduction:

subject01_walk1_ik.mot: Contains the time histories of model kinematics, including the joint angles and pelvis translations.

gait2354_RRA_Tasks.xml: A tracking tasks file specifying which coordinates to track and the corresponding tracking weight (weights are relative and determine how "well" a joint angle will track the specified joint angle from IK). Use this file to additionally specify any constraints on the RRA actuators. Two key considerations:

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.

gait2354_RRA_Actuators.xml: Ideal joint actuators used to replace muscles. The Actuator Set specifies the residual and reserve actuators to be applied and their parameters, such as maximum/minimum force and body, joint, or location (depending on the actuator type). A few key considerations:

1. Each degree-of-freedom (DOF) in the model should have an ideal torque or force (reserve) actuator. This includes the 6 DOFs of the model's base segment, which are called the "residual actuators".
2. In most circumstances, these ideal joint actuators are used to replace the muscles in the model (by checking "Replace model actuators" in the Actuators tab).
3. Optimal forces are the maximum output of ideal actuators (torques, linear forces). Torque (force) applied is optimal_force*control_value.
4. Residual at the pelvis should be applied at the scaled COM location.

Outputs

The Residual Reduction Algorithm Tool generates the following outputs:

subject01_RRA_states.sto: Adjusted kinematics (i.e., joint angles) and corresponding model states of the simulated motion (i.e., joint angles AND velocities).

subject01_adjusted.osim (optional): A model with adjusted mass properties.

subject01_RRA_controls.xml: Actuator excitations (i.e., control signals needed to generate actuator forces and torques).

subject01_RRA_Actuation ... (not shown in figure): Actuator forces and torques (i.e., joint torques corresponding to adjusted kinematics).

subject01_RRA_Kinematics ... (not shown in figure): Joint angles, velocities, and accelerations

subject01_RRA_avgResiduals ... (not shown in figure): Summary of the average residual values (FX, FY, FZ, MX, MY, MZ) for the trial

subject01_RRA_Kinematics ... (not shown in figure): Joint angles, velocities, and accelerations

subject01_RRA_pErr ... (not shown in figure): Position errors for each of the model's generalized coordinates during the trial

Best Practices and Troubleshooting