GRF in GaitFullBody model

Main Forum
1

Hello,

I wrote the following code lines in order to export the GRF, which are applied to the human in the GaitFullBody model (AMMR.v1.6).

Could you please check this, and tell me if it is correct?
I have some doubt because the first peak value of the GRF ( ~586.38 N) is less than the Bodyweight (~608.2 N).

///GRF Plate 1
///////////////
AnyForceMomentMeasure GRF_NetEffectMeasure1 =
{
AnyRefFrame& ref = .EnvironmentModel.Plate1.ForcePlate;
// AnySeg& seg = .BodyModel.Right.Leg.Seg.Shank;
AnyForceBase& Forces = .EnvironmentModel.Plate1.Force;
AnyForceBase& Moments = .EnvironmentModel.Plate1.Moment ;
AnyVec3 Flocal = Fref.Axes;
AnyVec3 Mlocal = Mref.Axes;
};

  AnyOutputFile GRF_NetEffectMeasureGlobal_1=  {
    FileName = "GRF_NetEffectMeasureGlobal_1.txt";  //name of the output file
    NumberFormat = {
      Digits = 15;  //number of digitsd
      Width = 22;  //number characters in number
      Style = ScientificNumber; 
    };
    SepSign = ";";  //separator sign between columns
    LineSepSign = "";  //separator sign for line shifts
    Header =    {
      TitleSectionOnOff = Off; //swicth the header on/off
      ConstSectionOnOff = Off; //swicth constan section on/off
      VarSectionOnOff = Off;   //swicth the variable on/off
      ColumnNamesOnOff = On;  //swicth the column names on/off
      LinePrefix = "";
    };

    AnyVector F_GRF_NetEffectMeasureGlobal_1 =.GRF_NetEffectMeasure1.F ; 
    AnyVector M_GRF_NetEffectMeasureGlobal_1 =.GRF_NetEffectMeasure1.M ;   
  };

///////////////

Cheers!

2

Hi

Your code appears correct, you will see the same kind of data if you look directly at the forceplate data in the forceplate classes.

I think the difference could come from two reasons:

[ul]
[li]Bodymass could have been less than 62kg in this experiment
[/li][li]Calibration matrices from the forceplates could be slightly off
[/li][/ul]

We will try to address this in the next release of the models.

Thanks for bringing this to our attention

Best regards
Søren

3

Hi Søren,

Thank you for your answer. It brings me to another question.
How does AnyBody perform the calculations and converges to a solution with a mass (Body weight), which does not correspond to the GRF provided by the forceplates ?

kind regards,
Arnaud

4

Hi Arnaud,

For mocap model there is usually a residual force applied in the pelvis.

Such residuals will always be needed to some extent, due to measurement errors, differences in anthropometrics etc.

You will find this residual if you look at in the drivers file for inverse dynamic model, here the reactions on the pelvis global position is on, and not off.

Best regards
Søren

5

see also

http://wiki.anyscript.org/index.php/All_about_Kinetics#Why_is_it_ok_to_have_a_reaction_force_between_the_pelvis_and_environment_if_you_have_measured_ground_reaction_forces.3F

6

Hi Søren

thank you again for your answer!

Cheers
Arnaud