Defining the variables

To define the variables for the Vicon ProCalc force vector tutorial, follow these steps:

Load the trial

Before you begin, ensure you have unzipped the  ProCalc Tutorials data as described in About the data for this tutorial.

For this tutorial, we will be using a static trial with a lower-body marker set where the subject stands on a force plate in a static pose. 

1. Start ProCalc
2. On the Data Management tab, navigate to the Force Vector session folder.
3. From there, double-click Staticto load the trial.
   The following data is displayed in the 3D workspace:
   
   

Define the force vector as a line in ProCalc

To define the force vector as a line in ProCalc:

1. Create a new Variable Scheme.

2. Add a new variable called ForceLine1 as shown below:

   

3. Observe that a line appears in the 3D workspace. This line can now be used for calculations.

Note that we could not use the force vector directly because ProCalc creates two new variables automatically when reading the force plate data from the C3D, namely a Point, which corresponds to the CoP, and a Vector, which corresponds to the direction and magnitude of the force vector (where the magnitude of the vector, in millimeters, corresponds to the force in Newtons, for example, an 800 N force is visualized as an 800 mm long force vector).

However, a vector in ProCalc does not have a fixed position in space, only a direction (this is the standard mathematical definition of a vector). A Line, on the other hand, is defined as having a direction and a position, and this is why we have to define the force vector as a line before we can do further calculations.

Define the variables to which to compare the ForceLine1 variable

Next, we define the variables to which we want to compare our ForceLine1 variable.

The variables to which we want to compare the ForceLine1 variable are the two feet and the pelvis.

To define these variables:

1. Add a new variable called LeftFootLine using the function From point A to point B, and A=LHEE and B=LTOE.
2. Click Copy and Mirror to quickly define the equivalent line for the right side.
3. Add a new variable called ASISLine using the points LASI and RASI.

We have now defined all the lines we need for our further calculations, and we can proceed to the lines that we will use for our results.

Define the lines to be used for the results

1. Add a new variable called LeftFootLine_to_ForceLine1, and choose the Line function: The shortest line connecting line A and line B.
2. Choose A = ForceLine1 and B = LeftFootLine.
   The line connecting these two other lines is displayed in the 3D workspace.
3. Copy and Mirror to define the equivalent for the Right side.
4. Add a new variable called ASISLine_to_ForceLine1, equivalent to the above.

Define the line lengths

Finally, we need to define the actual numbers we're interested in, which are the lengths of these lines:

1. Add a new variable called LeftFootLine_to_ForceLine1_Dist, choose function group Distance, and the function Magnitude A. Choose Type = Line, then choose A = LeftFootLine_to_ForceLine1.
   This measures the distance between the CoP and the left foot's line.
2. Copy and Mirror to get the equivalent for the right side.
3. Add a new variable called ASISLine_to_ForceLine1_Dist in the same way, using the ASISLine_to_ForceLine1 variable.

The following variables are now defined:

Before continuing, remember to save the scheme.