Apply External Loads to Shoulder

[Alex Denton]

Hello,

I am trying to incorporate isometric 6DOF force and torque data obtained by a custom build Biodex machine during a static optimization analysis.

I am familiar with setting up GRF external loads using this tool, but there are some difference I am having trouble figuring out.

I am using the thoracoscapular shoulder model found here: https://simtk.org/projects/thoracoscapular.

I have my .xml file ready to go with the experimental data - this includes force and torque data, as well as isometric joint angles. I want to apply these forces and torques to the shoulder so during the set up of external forces, I selected humerus for "applied to". I also am assuming I want this external load at the shoulder to be a body force rather than a point force, but please correct me if I am wrong. Then I select the appropriate columns for force columns and torque columns.

This next step is a little confusing to me. I'm not sure what to select for "Force Expressed in" and "Point Expressed in". If this were GRFs, it would be ground, but because these forces and torque are being applied to the humerus, I'm not sure what they should be expressed.

Thank you in advance. Please let me know if additional information is needed.

[Thomas Uchida]

I also am assuming I want this external load at the shoulder to be a body force rather than a point force

The difference between a point force and a body force is described in the documentation here: https://simtk-confluence.stanford.edu:8 ... cification.

I'm not sure what to select for "Force Expressed in" and "Point Expressed in". If this were GRFs, it would be ground, but because these forces and torque are being applied to the humerus, I'm not sure what they should be expressed.

"Force/Point Expressed in" is the reference frame with respect to which your data is expressed. The fact that the forces and torques are being applied to the humerus is irrelevant.

[Alex Denton]

Hi Thomas,

Thank you for your reply. I have previously read the linked site many times but it does not clarify when to use point or body force. I also do not see why it is irrelevant that forces and torques are being applied to the humerus. I'm not sure which to use, and what to select for "Force Expressed in" and "Point Expressed in".

[Thomas Uchida]

it does not clarify when to use point or body force.

Whether you select "point force" or "body force" depends on how your measurements have been expressed---specifically, whether or not the point at which your external force should be applied is at the origin of the body. "Body force" is appropriate only if the force is to be applied to the origin of the body; otherwise, you should apply a "point force" and specify the point of application on the body.

I also do not see why it is irrelevant that forces and torques are being applied to the humerus.

Here is the complete statement from my previous post: ""Force/Point Expressed in" is the reference frame with respect to which your data is expressed. The fact that the forces and torques are being applied to the humerus is irrelevant." Whether you are applying a force to the femur, humerus, etc. is a separate issue from the reference frame with respect to which the force vector is expressed. For example, if you apply a constant force of [1,0,0] and you indicate that the force is expressed in ground, then a 1N force pointing in the direction of the global X-axis will be applied (regardless of the body to which you are applying it). If, on the other hand, you say that the constant force of [1,0,0] is expressed in the humerus frame, then a 1N force will be applied (to whichever body you have selected), but it will point in the direction of the humerus body's x-axis.

[Alex Denton]

I'm still working on this problem and thought I would give an update.

The "Force Expressed in" and "Point Expressed in" should be the COM of the humerus, and the force should be a point force. After adjusting my external loads file, I re-ran the static optimization, however the results for the muscle activation seem a little noisy. I'm not sure how to tell if my results are decent.

After the static optimization, I tried to "Associate Motion Data" however I'm not seeing a force vector at the humerus. I'm only seeing a small green pentagon near the inside of the center of the rib cage. If I did this correctly, should I be seeing a force vector applied to the COM of the humerus?