Step 2 - Identify contact surfaces

When calculating joint kinetics, the Ground Reaction Force (GRF) measured by a force plate is used to estimate the force exerted from the ground onto the body. However, in order to determine which body part is in contact with which force plate, the system needs a way to determine whether or not there is a valid contact.

ProCalc 1.6 adds a new data type to help with this called a Surface. A Surface is simply a rectangle that has a location and orientation in space; for example, the top surface of a force plate is in itself a Surface.

To define a new Surface:

1. Navigate to the Variables tab and create a new Variables scheme.

2. Add a new Variable called LeftFootSurface.

3. Select the function type as Surface and the function as Define Horizontal Contact Surface from points A, B, C, margin D. This function makes it easy to create a Surface that roughly corresponds to the footprint of a person wearing at least three markers on the feet. 
   
    
   
   

4. For the Input Variables, select LHEE, LANK, and LTOE and the default length of 1mm.
   
   
   
   This creates a horizontal surface based on the locations of the three chosen markers, which is then the horizontal footprint surface of these markers at the height of the lowest marker – optionally with a bit of extra margin (the margin in this case is 1 mm. You can increase it by entering a different Factor, for example, 10 mm margin = 1 mm x 10).
   At this point, the Surface is displayed in the 3D workspace, assuming the three markers are present in the current frame.
   
   
   
   

5. Click Copy and Mirror to create a surface for the Right Foot.