Step 3 - Identify limb contacts

At this point, we are ready to start identifying when a limb comes into contact with a force plate so that we can associate the ground reaction force. In this example, the limb contacts we are interested in are foot strikes.

To identify the foot strikes:

1. Add a new variable and name it LeftFootReaction.

2. Select the function type Kinetics and the function Force plate contact from Surface A, max distance B, max Speed C.

   

   This function connects the Surface we defined in the last step with a force plate – if the distance of the Surface to the force plate is less than the maximum distance B, and the Surface has a maximum speed C. You could, for example, enter the following:

   

   The interpretation of this is:
   A valid force plate strike is registered if the surface (LeftFootSurface) is less than 150 mm to the plate AND is moving at a speed lower than 2500 mm/s (2.5 m/s).

A good force plate strike is usually characterized by a few criteria:

• The foot markers are all above the plate.
• The foot markers not too far above the plate.
• The foot markers are stationary or moving slowly.

To improve the last two criteria, you can use relative rather than absolute values.

To improve foot strike identification by using relative values:

1. Create a new variable called LeftFootHeight.
2. Select the function type Distance and the function Distance: A to B.
   

3. Change the Input Variable Type to Surface for both A and B. Specify Left Foot Surface for A and Floor Surface for B.

   

   This calculates the distance between the foot (surface) and the floor. This calculation assumes that the floor was calibrated to be flat and its values should have its lowest points when the foot is on the floor.

4. Create a new variable called LeftFootSpeed.
5. Select the function Velocity and the function Velocity Magnitude: A

6. Change the Input Variable Type to Surface and specify the LeftFootSurface as the Input Variable.

   

7. Edit the variable LeftFootReaction from earlier.
8. Edit the Length Input Variable to Left Foot Height and the Number to LeftFootSpeed.
9. Edit the Factor for B to 0.5 and for C to 0.2.

10. Edit the Timing for B and C to be Maximum.

    

    We are now using two variables as criteria in the foot contact determination instead of fixed values. What is important here is that we evaluate the Maximum. This means that ProCalc evaluates the input variable to its maximum for all frames. We can then apply a factor – in this case, the foot strike must have a foot height of no more than half of the maximum, and a foot speed of no more than 20%. In this way, we can use relative values as evaluation criteria, to account for variability between subjects.

11. Copy and Mirror LeftFootHeight, LeftFootSpeed, and LeftFootReaction.