Load path during standing

[naga (NAGARAJAN S)]

Hi,
I have imported a .bvh file of a subject standing using data captured from Xsens. I have been able to compute the inverse dynamics for the same. I am checking the ProximoDistal Joint reaction forces at different regions from head to foot (C7T1, T12L1, L5S1 followed by hip, knee and ankle joints). Kindly clarify the three points regarding the joint reaction forces.

1. There is a huge difference in the joint reaction forces when the muscles are switched off and switched on. What may be the reason?
2. The mass of the head and neck correlates to about 8% (from standard literature) of the Body weight which closely correlates with the C7T1 proximodistal force when the muscles are switched off and increases by 4 times when the muscles are switched on.
3. The vertical component of the GRF prediction model does not match with the vertical compontent of the proximodistal forces measure at the ankle joint.
   My aim is to understand how the self weight of the body is being transferred through the various joints during standing.
   Thanks in advance

[Bjorn (Bjørn Keller Engelund)]

Hi @naga

Joint reactions are very different depending on whether you have muscles in the model or not.
If you only have joint torque providers in the model (i.e. no muscles), then the joint reactions are only what the external forces produce.
When you include the muscles, their contribution to the joint reaction is added and it is often larger than what the external forces provide.
A muscle can only produce torque in a force couple with a joint reaction, so the real joint reactions are much larger than what you get from a model with pure joint torques.

In other words, it is correct that you get different joint reactions depending on whether you include muscles in the model or not.

Best regards,
Bjørn