Inquiry about column headings in subject01_walk1_grt.mot file

[Silas Purja]

Dear OpenSim Forum Members,

I hope this message finds you well. I am currently working through the OpenSim Inverse Dynamics tutorial (Tutorial 3), and I have come across a file named subject01_walk1_grt.mot. In this file, there are several column headings, and I am seeking clarification on their meanings. I would greatly appreciate it if someone could provide insights into the interpretation of each of the following column headings:

Why do you have "1" in front of some of the names? is it to differentiate between right and left leg?

ground_force_vx
ground_force_vy
ground_force_vz
ground_force_px
ground_force_py
ground_force_pz
1_ground_force_vx
1_ground_force_vy
1_ground_force_vz
1_ground_force_px
1_ground_force_py
1_ground_force_pz
ground_torque_x
ground_torque_y
ground_torque_z
1_ground_torque_x
1_ground_torque_y
1_ground_torque_z


Any assistance in understanding these column headings would be invaluable to my progress in the tutorial. I am eager to enhance my understanding of OpenSim, and your expertise will be of great help.

Thank you for taking the time to read my inquiry, and I look forward to any guidance or explanations you can provide.

Best regards,
Silas

[Nicos Haralabidis]

Hello Silas,

They correspond to measurements from different force platforms, and you can see how those prefixes are applied in the external loads setup file:

Code: Select all

subject01_walk1_grf.xml

For example, the measurements corresponding to 1_ground_* are applied to the left calcaneus in this example. You could also hypothetically have a third force platform, with prefix 2_ground_*, and you could apply this force to the limb which contacted the plate.

Hope that helps!

Nicos

[Silas Purja]

Hi Nicos,

I hope this message finds you well, and I appreciate your prompt and informative response to my previous inquiry.

In Tutorial 3, while visualising the ground reaction force, I observed the representation of the ground reaction force as being a green arrow with magnitude and direction. I noticed that the arrow's base starts from the heel, progresses towards the center of the foot during the stance phase, and eventually reaches the toe at the toe-off part of the stance phase.

I have a couple of questions:

1) What exactly is the point of application (ground_force_px, ground_force_py, ground_force_pz)? Could you kindly explain what these columns represent in terms of the point of application during the gait cycle?

2) Regarding the XML file you mentioned, I have a question about the line below:

<!--Name of the body the force is applied to.-->
<applied_to_body> calcn_l </applied_to_body>

So my question is that these two lines above indicate that the force is applied to the body "calcn_l," referring to the left calcaneus. What does this exactly mean? and I am unclear about how this corresponds to the observed movement of the ground reaction arrow from the heel to the toe during the stance phase. Could you elaborate on what is meant by "force applied to calcaneus," especially in the context of the dynamic movement observed during the gait cycle?

I greatly appreciate your time and expertise in helping me understand these concepts better.

many thanks,
Silas