_Joint Reactions Best Practices

Feb 06, 2012

2 min read

If JointReaction doesn't recognize a specified joint, body, or frame name, it will perform its default action. The overall default action is to report the loads on all joints as applied to the joint's child body and expressed in the ground reference frame. 
Consistency of modeling inputs to the Analyze tool: The validity of the joint loads depends on modeling assumptions and correct modeling practices. JointReaction is very sensitive to the consistency of all inputs that define a dynamic trial, including the following inputs to the Analyze Tool: <model_file>, <replace_force_set>, <force_set_files>, <states_file>, <coordinates_file>, <speeds_file>, <lowpass_cutoff_frequency_for_coordinates>,<external_loads_file>, <external_loads_model_kinematics_file>, <lowpass_cuttoff_frequency_for_load_kinematics>
If any of these files are not associated with the same dynamic trial, the system of accelerations and forces will not be consistent. Therefore, JointReaction will calculate incorrect joint loads. 
Special Types of Forces: Certain types of forces and actuators, called SpringGeneralizedForce, CoordinateLimitForce, and CoordinateActuator, are associated with specific degrees of freedom and treated as part of the joint structure. This means that any contribution from these components will be treated as part of the resultant loads reported by JointReaction instead of body forces. As an example, consider a reserve actuator on the hip joint of a gait model. If this reserve actuator is defined as a CoordinateActuator, its contribution is treated as a residual force provided by the joint and therefore will be added to the resultant load at the hip. However, if the hip reserve is defined as a TorqueActuator, its torque will be treated as a motor external to the joint and attached between the pelvis and femur. Therefore, its torque will not be added to the reported resultant load at the hip.

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