Drop Landing Model as example for Legpress Simulation

[Günter Schneider]

Hi,

i wrote my question at the forum of the "Drop Landing Model"-Project, but didn´t get reponse. So I´ll try it here again, in the hope to get answers.

https://simtk.org/forums/viewtopic.php?f=809&t=3851

THANKS in advance

Günter

[Ayman Habib]

Hi Gunter,

Contact only guarantees that objects that are defined as contact geometry (in the same ContactForce specification) do not go through each other. In the drop-landing example this happens naturally because the contact spheres are at the bottom of the feet and the forces from contact end up either pushing the model up to bounce or let it slide on the floor. If you can post a screenshot of what's going on with your model that you didn't expect we can help you troubleshoot further.

Best regards,
-Ayman

[Günter Schneider]

Hi Ayman,

thx for your response.
My aim is/was to situate my model on the plyo_block from the 3D Model Library. As rough approach of a legpress to get the forces in y direction. My consideration was that body forces due to gravity affects the cmc results of my legpress motion especially rect_fem.

For example activation of rect _fem is directly 1 (figure 1: V0-6), when I run CMC without vertical forces (+ y axis) and the 2% of the measured/experimental forces in horizontal direction (- x axis).

figure 1 (Fy=0, Fx=-15N)

When I run CMC with vertical forces applied at calcn left and right (50N in + y direction) and the 2% of measured forces in horizontal direction (- x axis) activation of rect_fem decreases earlier when knee angle increases.(figure 2: V0-7)

figure 2 (Fy=50N, Fx=-15N)

So my speculation is that activation of the rect_fem is that high because of the forces due to gravity.

That´s why I thought, when I place my model on a "legpress" I can eliminate this forces and get net activation. I read your answer to the question of thomas (https://simtk.org/forums/viewtopic.php?f=91&t=3845) and so I think it´s not the best way to solve that problem? Isn´t it? Does gravity, or the forces due to gravity infect the cmc results? I do think so? As you said in your answer by adding them into the grf file?

THX again, and best regards

Günter

edit: I had some mistakes in the files I attached, SORRY

[Ajay Seth]

What do you mean by does gravity "infect" the solution of CMC? If gravity is present its force is always exerted. You can set gravity to zero if you wish to simulate a leg press in microgravity. If your legs are horizontal and unsupported, do you not need muscle forces (e.g. rectus and other hip flexors) to keep them up? Either muscles or the frictional force and normal force (if sloped) of the leg press foot stops need to support the legs against gravity. Were you expecting the footstops to support more of the leg weight?