Step 5 - Define parameters

Calculating parameters is the final step in the ProCalc workflow. Often, either for research or clinical purposes, it is useful to extract particular parameters from the captured data. These might be parameters that are being investigated as part of a research project where statistical calculations are performed on the output, or they might be clinically relevant parameters that are compared with “normal” parameters, to ascertain how much the patient's movements deviate from them.

To define parameters:

1. In a similar way to the other scheme types, click on the Parameters tab, click the Create button , name the scheme and press Enter.

2. Add a new parameter by clicking the Add button .
   A screen similar to the following is displayed:
   
   

3. We will first extract some parameters from the variables we defined earlier, at the events we also defined earlier. To do this:
   
   1. Look at the Function drop-down menus – the first contains the function groups, the second contains the function. For your first parameter, select the Value group and the Variable A [Event A] function. This simply calculates a specific variable’s value at every occurrence of the specified event. Already you see one of the key concepts of parameters: they are not necessarily unique for a trial, there could be multiple occurrences of them, depending on the combination of variables and events.
   2. From the A Type list, select EulerAngle, and then from the Input Variable list, select PelvicAngles. From the XYZ list, select X, and from the Input Events list, select LeftMidSwing.
   3. Name this parameter LeftPelvicAngleMidSwing, and look at the calculated parameter’s value in the log to ascertain if it looks reasonable or not.
   4. Note that you can also change the output unit – in this case, you can change the default deg (degrees) to rad (radians).

   5. As for the other schemes, you can also click the Copy button  and the Mirror button  to create the equivalent parameter for the other side.
      You can use many different functions when defining parameters. Try them out in the interface, or for a full description, see Functions for defining parameters in the Vicon ProCalc Product Guide.

To explore other parameters, try the following:

• Click the Add button , name the parameter, then for the Function group, select Time, and the Event A option.
  This returns the time of Event A, which is perhaps not very interesting in itself. However, if at the bottom of the interface you select Time Normalize Between and select two events, you could, for example, investigate whether the mid-swing event really happens half-way between the foot off and the foot contact event – simply select the check box and choose from the drop-down menus.

• Click the Add button , then select the Total time group, Above Value [Event A, Event B] and the type Point, the variable LHEE, and the component Z.

  1. In the Threshold Value field, enter 50 and select % cycle max from the drop-down menu.
  2. For events A and B, select Left Foot Strike (for both events)
  3. Select Time Normalize Between, and select the same two events.
  This calculates the percentage of the gait cycle where the LHEE marker’s Z coordinate is at least half the value of the LHEE marker’s maximum Z value for the current cycle (to use the overall maximum Z coordinate instead, simply change the threshold value’s definition to % trial max).

• Click the Add button , name the parameter, then select the Average function group, the Variable A [Event A, Event B], then EulerAngle, PelvicAngles and X.
  For input events A and B select Left Foot Off and Left Foot Strike
  The average pelvic tilt during swing is returned.

• Click the Add button , name the parameter, then select the 3D function group, the Distance Travelled Variable A [Event A, Event B], then Point, LANK, XYZ and Left Foot Strike to Left Foot Strike.
  This returns the total distance traveled by the LANK marker during the gait cycle along the path of the marker’s trajectory. (To calculate the straight line distance, select the Distance Variable A [Event A] to Variable B [Event B] and for both A and B, specify LANK).

• Click the Add button , name the parameter, then select the Temporal function group, then Double Support [Event A, Event B] – [Event C, Event D].
  Double support is defined as the amount of time both feet are on the ground as a percentage of the gait cycle, and for the left side this corresponds to the stance time (foot contact to foot off) minus the right side’s swing time (foot off to foot contact). Therefore, for A, select Left Foot Strike, for B, select Left Foot Off, for C select Right Foot Off and for D select Right Foot Strike. In other words, A->B is ipsilateral stance whereas C->D is contralateral swing.

Parameter types

When you define parameters, you can choose what you want to use the parameter for. There are three initial choices:

• Export parameter – The parameter is exported to Microsoft Excel when the scheme is processed.
• Model parameter – The parameter is calculated and stored internally to be used for later processing. This is especially useful if your calculations require certain offset values to be calculated from a static trial, which will be used in the dynamic trial.
• Internal parameter – The parameter is used as an intermediate step for calculating other parameters.

To choose the parameter type, select the Model Parameter or Export Parameter check boxes. If neither of these is selected, the parameter is an internal one.