# how inversedynamics work

Hi, AnyBodySupport

There’s a question that how inversedynamics work?
1.
For example, when we simulate the walking model, we need a force data from global to foot, and use inversedynamics to analyze.
If there’s no force data, just hook the human model and the environment, in this situation, is the output value is right? Like RectusFemoris, GastrocnemiusLateralis or other lowerbody muscle.

And if there’s a baseball pitcher model simulate after motion capture, and we are interest in upperbody muscle, like Brachialis, Triceps…If there’s no forceplate or close the lowerbody, in this situation, will the upperbody muscle activity value can not be use?

Thanks first
Yeh

Dear Yeh,

A1: If you have the connection between the pelvis and the world the results will not be correct for the legs, it will be like hanging in the air.

A2: Everythings is connected in such a model, but i think the arm muscles will be ok.

If you are mainly interest in the arms and have no forceplates, i would remove the connection the pelvis? and make an reaction force between the grounds and the feets instead.

Best regards
Søren

Dear Søren

Thank you very much.
After reading in your explanation, I learn that if I’m interest in lower body, I absolutely need a force data from environment to feets.

But if I’m interest in upperarm or shoulder, I have two ways to do.

1. If I close the lowerbody to 0, just using markers simulate the model (maybe there’s a connection between pelvis and world.), because the upperbody inversedynamics is calculate from finger to arm to pelvis?
2. But if I have the lowerbody in the model, I should remove the connection betweeb pelvis and the world, then make an reaction force from grounds to feets.

If my understanding is wrong or something needs to fix, I’ll very appreciate you to correct me.

Best regards,
Yeh

Hi Yeh,

Some small corrections:

If you have always two feets on the ground i would make an AnyReacForce between them and the ground, and also remove the reaction in the pelvis.

1: if you have a model with no legs you would use a connection between the pelvis and the world. The markers would provide the motion. If you are mainly interested in the upperbody this would be an ok model.

2: yes that is correct otherwise they would just hang there unloaded.

Best regards
Søren