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

## Load a suitable trial

For this tutorial, you can use any static trial with a lower-body marker set where the subject stands on a force plate in a static pose.

- If you do not have a suitable trial to hand, download the Vicon Nexus Sample Data from:

https://www.vicon.com/software/models-and-scripts/nexus-sample-data/?section=downloads - Install the sample data.
- Navigate to the session:
- From there, load the
**Cal 01**static trial in ProCalc.

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:**

- Create a new Variable Scheme.
- Add a new variable called
**ForceLine1**as shown below: - 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* corresponding to the CoP, and a *Vector* corresponding 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:**

- Add a new variable called
**LeftFootLine**using the function**From point A to point B**, and A=**LHEE**and B=**LTOE**. - Click
**Copy**and**Mirror**to quickly define the equivalent line for the right side. - 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

- Add a new variable called
**LeftFootLine_to_ForceLine1**, and choose the Line function:**The shortest line connecting line A and line B**. - Choose A =
**ForceLine1**and B =**LeftFootLine**.

The line connecting these two other lines is displayed in the 3D workspace. **Copy**and**Mirror**to define the equivalent for the Right side.- 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:

- 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. **Copy**and**Mirror**to get the equivalent for the right side.- 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.