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Knee Joint Example

Using the OpenSim GUI, one can select the parent and child joint frames of a joint and modify their values to see how the joint's definition changes.

A simple and illustrative example is the knee joint on the RCNL Model. For this example, we can use the RCNL Model found in the RCNLModel directory of nmsm-examples.

  1. Load the RCNL Model in the OpenSim GUI.
  2. Locate the knee_r joint in the left menu.
  3. Uncollapse the joint to show its parent and child frames.
  4. Select the child frame.
  5. In the bottom left menu, double click the rotational joint parameters to modify the first value (X rotation) value.

Notice how the knee adduction angle changes?

Next, modify the third value in the child frame's translational joint parameter (Z translation).

Did the tibia shift laterally away from the femur?

Example Takeaways​

The first modification, knee adduction angle, modified a joint parameter that varies between subjects and contributes to inverse kinematics marker distance errors and the quality of other computations. The second modification may also improve inverse kinematics marker distance errors, but through non-anatomical modeling. Modifying the Z translation of the knee joint should be excluded because it could cause JMP change joint parameters that no longer describe the joint in a physically possible way.

Modifying Joint Parameters Along Coordinate Axes​

Another concern for selecting joint parameters is selecting parameters that rotate along a free coordinate in the model. Modifying the above example's knee_r parent frame Z rotation value will have no effect on the resulting JMP results because the model can change its coordinates to find the exact same inverse kinematics results regardless of the joint parameter value. Adding these joint parameters to the optimization will cause the tool to run slower and could cloud the ability of the optimizer to find a global minimum.

Modifying Joint Parameters Without Sufficient Range of Motion​

Some joint parameters cannot be accurately or sufficiently optimized if the range of motion of the joint of interest is not sufficiently large during the trial used for JMP. A good illustration of this is the parent X translation of the hip during a gait trial. Since the hip does not adduct sufficiently during gait, finding an accurate position of the hip joint along the X axis is not feasible. An estimation will be made, but there is low confidence in the result if the range of motion of the joint is less than ~25 degrees.

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