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Ground Contact Model Personalization

Tutorial Developers: Robert Salati, B.J. Fregly, Rice Computational Neuromechanics Lab, Rice University

Last Updated: 3/10/2026

In this step for the soccer kicking example, you will use the Ground Contact Model Personalization (GCP) tool to personalize a ground contact model for the subject's left foot. To perform the GCP process, you will be using the osim model Walking_Model_jmp.osim that you personalized in the previous step.

The kicking motion is cropped so that the trial starts only after the subject jumps off the ground. The kicking (right) foot is never in contact with the ground during the trial, and so we only need to a ground contact model for the right foot.

Unfortunately, we do not have good ground reaction force and moment data for this trial, so we need to be creative.

Question

Think for a few minutes how you could personalize a ground reaction model to a subject for a specific trial without ground reaction data from that trial.

The solution is to use ground reaction data from a different trial for the same subject wearing the same shoes in the same lab setup. In this case, we are using gait motion for the same subject, but in the same lab setup. Therefore, we can assume that the ground reaction models should be approximately the same between the walking and kicking motion.

Question

What are some limitations you can think of for using walking data to personalize a ground contact model for a kicking motion? Think about what the main differences are between walking and kicking. How do the speeds of the motions and the ground reaction force magnitudes compare?

The data used in this tutorial are:

  • Trial12_Gait_forces_filtered.mot
    • Processed 6 degree of freedom ground reaction data in electrical center format from a walking trial.
  • Trial12_Gait_IK_results_filtered.mot
    • Processed IK data using Walking_Model_jmp.osim.

Setting up a GCP settings file.​

  1. Activate the NMSM GUI in OpenSim by navigating to Tools>User Plugins, and click rcnlPlugin.dll.

  2. With Walking_Model_jmp.osim selected in the OpenSim GUI, navigate to Tools>Model Personalization>Ground Contact Personalization.

  3. Leave the Osimx file field empty.

  4. Set the Motion file to Trial12_Gait_IK_results_filtered.mot.

  5. Set the ground reactions file to Trial12_Gait_forces_filtered.mot.

  6. Set the results directory to GCPResults.

  7. Add a new contact surface

    • Set the name to "Left Foot"
    • Check Left Foot
    • Set the time range to 1.5 - 2.56
    • Set the belt speed to 1.2
    • Set the force columns to ground_force_1_vx ground_force_1_vy ground_force_1_vz
    • Set the moment columns to ground_moment_1_mx ground_moment_1_my ground_moment_1_mz
    • Set the electrical center columns to ground_force_1_px ground_force_1_py ground_force_1_pz
    • Set the hindfoot body to calcn_r
    • Set the markers as follows:
      - Toe Marker: R_Toe
      - Medial Marker: R_Toe_Medial
      - Heel Marker: R_Heel
      - Lateral Marker: R_Toe_Lateral
      - Midfoot Superior Marker: R_Midfoot_Superior

  8. Save the settings file as GCPSettings.xml

  9. Open up GCPSettings.xml in a text editor of your choice and explore the settings file.

  10. With a text editor, scroll to the bottom of the settings file, and change <max_iterations> to 20.

  11. With the text editor, change the <grid_width> and <grid_height> to 3 and 8, respectively.

    • To make the GCP run faster, we use a less dense grid of contact elements.

Running GCP​

Open runGCPTool.m in Matlab and click run

- Make sure the NMSM Pipeline Project file is open
Runtime

This GCP run will take 58 iterations to finish.

Analyzing GCP Results​

Plots should automatically be created by the runGCPTool.m script

Questions
  • What were the RMS errors for the ground reactions?
  • What were the RMS errors for the foot kinematics?
  • Which ground reaction and which foot kinematic quantity had the worst tracking error? How are these two quantities related to each other?
  • Try changing <grid_width> to 7. How does the ground reaction force and moment tracking change? How do the individual spring stiffnesses change? Does this make sense?