General Settings
Output Section
Analyzers
in Detail
Time
Rate
Percent
Value
Distance
Max Min
Mean
Area
Slope
To make a measurement, you must first define an "analyzer". There are
several different analyzer types, which make different, simple, measurements
for each matching instance of events and trajectories.
When creating a new analyzer, a tabbed dialog appears which shows you all
the alternative types. Choose the one you want, fill in the details, and
press "Okay".
You can edit the values later by choosing the event in the event list, and
pressing "Edit".
Events
You need to know the name of the events that you want to find. If you generated them using the Event Correlator, then you should use the same name. Events inserted in Vicon Workstation are called "Foot Strike", "Foot Off" and "Event" (even though it's referred to as a "General" event). The Workstation events and any events that you have already defined for use in the EventCorrelator PlugIn will be listed in the drop-down list which you can choose from. These fields are not case sensitive, but spaces are important.
For each event, you can choose individually the Context in which it should be searched for. For example, you can define an analyzer to take a measurement based on a Foot Strike on the Left side, followed by a Foot Off on the Right side.
The analyzer will search for each event in the order and in the context you specify.
You can use the same name for each event, and the analyzer will give the result between each successive pair of events (e.g. from Left Foot Strike to Left Foot Strike).
Trajectory Label
You should just type the basic name of the trajectory label here. Note that this field is case sensitive. You can use the drop-down list or you can type the name. The auto-complete mechanism will try to complete the name as you type with a matching label if it can find one. If it gets it wrong, just keep typing.
The drop-down list of trajectory labels is based on a marker set file which can be defined under the Cycles tab of the main settings dialog.
Trajectory Component
For analyzers which make measurements from trajectory data, you can choose which component of the 3D vector position of the marker you want to take.
Description
You can type in any description you like for the measurement you are making. This description appears in the output so you can tell what the measurement refers to. There is no other significance attached to this.
Units
Like the Description, you can type whatever you like here, and it will appear in the output. It does not alter the calculation. This facility is primarily designed for calculations based on trajectory data whose values do not represent real spatial quantities, but the results of previous modelling stages such as scalar, angle, force moment or power values. There is a drop down list of common alternatives to choose from.
Context
For each analysis result, you have the option to refer the result to a particular Context, which affects how the result is shown in the text file or in Polygon. Normally the context will be the same as that of the events, but this may not be true, depending on what it is you are measuring.
Don't output (only use for calculations)
The default setting is that the result of any analysis should be output (according to the settings). This can be overriden for any of the analyses by checking the "Don't Output" option.
This option allows you to generate parameters which are used in a subsequent calculation stage without cluttering the output with unneccesary data.
The Time analyzer simply measures the time between a pair of events.
Define the the First Event as "Foot Strike" and the second as "Foot Off" to
measure the period of time the foot is in contact with the floor.
The Rate analyzer is like the Time analyzer - but it calculates the
inverse of the period between the two events, and scales the result to be per
minute. Use the same event (e.g. "Foot Strike") for both the first and second
event to calculate the stepwise cadence.
The "Per cent" analyzer uses three events, and calculates the % of time at
which the second event occurs between the first and third events. Find the %
support time by defining the First and Third events as the "Foot Strike" and
the second event as the "Foot Off".
The value analyzer just extracts the chosen component of a trajectory at
the time a particular event occurs.
Find the Height of the toe marker at the "Foot Off" event, by selecting the LTOE marker, and choosing the Z component, or the total ankle force at push-off by selecting the LAnkleForce and choosing the Modulus component.
This analyzer can be especially powerful when combined with a customised modelling stage (such as PlugIn Modeller).
This measures the distance a marker moves between two events. You can choose to measure the straight line distance between the marker position at the two different times (i.e. the shortest distance) or you can measure the total distance the marker travels by integrating the distance the marker travels between each frame (the algorithm sums the straight line distance between each consecutive pair of valid points). For example, you could test a patient's ability to draw straight lines by defining an analyzer of each type, and comparing the distance a pen marker travels between a pair of General events.
Note that in the "Integrated Path" option, the analyzer only integrates the path when the points are valid for a marker, so you will get invalid results if the marker becomes occluded. However, this option gives you great flexibility if you want to measure distances that are the result of a modelling stage.
The "Max Min" analyzer calculates the minimum and maximum of a trajectory value during the period between the two given events. You can choose to output either, or both, and the range (i.e. Max - Min).
You can also choose to output the % time at which the maximum or minimum occured. This can be expressed as the percentage of the main analysis cycle (depending on your Cycle settings) or during the time period specified by the two events.
To find the peak knee flexion during the swing phase, choose Foot Off and Foot Strike as the two events, and select the KneeAngles trajectory (Left or Right depending on the context) and the "X Direction" component to get the first angular component.
Other possibilities are to find variables such as the Peak power during a
cycling revolution; minimum golf club height during the swing...
Use this analyzer to find the mean value of a trajectory variable between
a pair of events. Use a previous modelling process to calculate the velocity
of a marker, and use this analyzer to find the mean velocity on a cyclic
basis.
The Area analyzer performs an integration of the trajectory data using the
trapezoid method. This can be used in gait analysis for the integration of
power curves on a gait cycle basis (especially useful when combined with the
Event PlugIns to define an extended range of gait cycle events).
Naturally, the slope analyzer measures the slope of the given trajectory
data (based on the component you select) at a particular event. Note that
this is the slope that the given component of the trajectory makes on a
time based curve - it doesn't tell you the actual direction a marker
was travelling in (you need to set up three separate slope anaylzers, for X,
Y and Z if you want to do that...) !
The algorithm calculates the slope between pairs of points either side of the given event, and takes the average. You can control the number of pairs of points it uses by changing the value for "Frames each side". Thus, if this value is 3, it will calculate the slope using points at T+1 and T-1; the slope from points T+2 and T-2; the slope from points T+3 and T-3, and then take the average of the three (if any of the required points is invalid, it disregards that measurement and takes the mean of what it can measure).