Removing (unnessary) limbs to simulate a squat using FDK

[nd8 (Nghi Dang)]

Hello everyone,

Currently, I'm working on a subject-specific FDK-knee model that performs a squat. The model is supposed to be as slim as possible so I'd like to keep head, trunk and left leg only (only left knee will be used for FDK) and remove all the other limbs. The goal is to analyse the kinematics and stresses in the left knee, so the other limbs are not necessary for my work. While removing the limbs worked so far, some muscles overload during the FDK-simulation. When reducing the body mass to about half the original (subject) weight, the error did not occur again. For that reason I have the following questions:

1. Is it even possible to simulate a squat with only one leg and no arms that shows no difference to a model which does include both legs and arms (i.d. all limbs), so that the stresses in the left knee could be analysed? Or will the remaining leg carry the whole weight of the body similar to a one-legged squat? If it is possible, could you please elaborate how to implement that?

2. Does exluding the arms lead to lower weight and therefore lower stress in the joints during squatting (even when using both legs)?

The model uses the TLEM 2.1 model. The same drivers as in THA-KneeBendDemo in the AMMR repository were used. Removing the right leg lead to two missing constraints so I added two drivers constraining the rotation of the pelvis in x and y direction.

Thanks a lot!

[Bjorn (Bjørn Keller Engelund)]

Hi @nd8

Is it due to speed gains that you want to keep the model slim?

1. it is possible to obtain what you seek, but it requires that you know the external force applied by the remaining body. You then apply that force to mimic having the full body.
   if you don't have the external force you could include the whole model and then only remove the muscles of the arms, trunk and right leg. That will enable joint actuators instead. Make sure that you motion keeps the model in balance by keeping the COM over the feet.
2. Yes the model weight is reduced when excluding body parts.

Best regards,
Bjørn

[nd8 (Nghi Dang)]

Hi Bjørn,

Yes, it's mainly due to speed gains but also to keep the model simpler. Since I define a user-specified left knee (including some manual adjustments regarding the alignment and orientation of femur, tibia and patella), I wanted to avoid applying the same adjustements to the right knee to keep the symmetry.

Thank you for your help. I will follow your advice and include all body parts again.

Best regards,
Nghi