Skip to main content

Best Practices

Joint Model Personalization's nuance comes from the ability to include (or exclude) any number of joint, marker motion, and body scaling parameters for each task in a JMP run. Below are some tips on how to decide what to include.

Joint Parameter Selection​

Joint parameters are the twelve parameters that describe the relationship between the parent and child joint centers for a given joint. Each parent and child is described by three (X,Y,Z) translational and three (X,Y,Z) rotational terms that define the nature of the joint.

Understanding Joint Parameters​

Joint parameters are not usually considered when using OpenSim; most default joint parameters are selected using cadaveric data and modelers tend to not modify these values. Even so, using default model values fails to produce consistent results. Modifying joint parameters intelligently can dramatically improve the usefulness of the model and the accuracy of downstream computations.

See knee joint example for more details.

Modifying Joint Parameters Along Coordinate Axes​

Another concern for selecting joint parameters is selecting parameters that rotate along a free coordinate in the model. In general, the axis that a joint rotates about should not be modified by JMP. 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.

Lower Body Joint Parameter Selection​

It is heavily advised that users do not simply allow all joint parameters to be 'on', but rather think analytically about which joint parameters should be modified to create an anatomically realistic, accurate, and unique solution.

Below are tables describing the recommended joint parameters that should be included and excluded from each joint in the lower body of the RCNL Model. Keep in mind that these are just recommendations based on our experience. If your clinical problem calls for personalizing parameters that are excluded in these tables, feel free to explore!

Hip​

Frame/TransformationXYZ
Parent Translation✓✓✓
Parent Rotation🗴🗴🗴
Child Translation🗴🗴🗴
Child Rotation🗴🗴🗴

Alternatively, the hip can be personalized by scaling the pelvis. Scaling the pelvis can move the locations of the hip centers relative to the whole body, but still keep the femurs attached to the hip socket.

Knee​

Frame/TransformationXYZ
Parent Translation🗴🗴🗴
Parent Rotation✓✓🗴
Child Translation🗴🗴🗴
Child Rotation✓✓🗴

Subtalar​

Frame/TransformationXYZ
Parent Translation🗴🗴🗴
Parent Rotation🗴✓✓
Child Translation🗴🗴🗴
Child Rotation🗴✓✓

Ankle​

Frame/TransformationXYZ
Parent Translation🗴🗴🗴
Parent Rotation✓✓🗴
Child Translation🗴🗴🗴
Child Rotation🗴🗴🗴

Note that the child frame of the ankle in the RCNL model is not personalized. This is because the child frame of the ankle joint is the pretalus which only defines the orientation of the talus bone geometry. Therefore, personalizing those parameters has no impact on the motion of the ankle joint.

Marker Motion Selection​

Marker motion should be done selectively and intentionally since allowing the markers to move allow all axes will absorb IK error, but may cause the model to not represent the underlying experimental data or anatomy.

We can understand marker motion selection via a few examples:

  • The subject has larger than average thigh circumference. As a result, you allow the femur_r and femur_l markers to move in the X and Z directions, that is, radial to the major axis.
  • The scaled model pelvis does not appear to be scaled correctly. Allow the pelvis to scale and also allow the femur_r and femur_l markers to move in the Z direction to prevent the pelvis scaling from reducing the accuracy of the femur markers.

Body Scaling Selection​

Body scaling should be done when lower IK marker error is desired but anatomically, the parent and child bodies of a joint are positioned correctly. The best example of this is the femur head placement in the pelvis. Thus, rather than optimizing the hip joint, better results can be found by scaling the pelvis.