Considerations

Categories

Experiment setup

In the APASview it is possible to combine various types of data fx. ground reaction force with digitized stick figures. Before this is possible some considerations about experimental setup must be made.

To standardize the interpretation of ground reation force vectors for all kinds of experimental setup the APASview must have some information concerning the orientation and position of the forceplate and the configuration of the global reference system.

When digitizing movement a calibration frame is used to create a global reference system in which the digitized points are transformed into. The global reference system can be considered as a huge 3D coordinate system created within the viewed location. The 3D cordonate system consists of three axis called X,Y,Z.

The only parameter needed for configurating the global reference system is the Y+ directinon with respect to the X+ direction.
In the following example the X+ direction is from the right to the left in the picture, and the Y+ direction is directed to the right in respect to the X+. The Z is considered positive upward for all experiment setup.

Example:

Forceplate signal sample frequency

Sampling forceplate signals into the computer the sample rate can be crucial for correct interpretation. Normally the forceplate data is avaraged to fit the sampling rate of the video. Normally video is digitized in freqency of 60Hz (NTSC - US) or 50Hz (PAL - Europa). If the sampleing rate for the forceplate is fx. 1000Hz which is not unsual, the avaraging sample window would be 16.7 for 60Hz and 20 for 50Hz. The 16.7 sample window is not possible and therefor it will be rounded up to 17. This means that the avagared forceplate will be missplaced in respect to the actual picture. To avoid this problem use a sample frequency of fx. 1200Hz for each channel in 60Hz mode or a sample frequency that will create an integer.

To calculate the avaraging sample window just divide the forceplate sample frewuency with the video sample frequency.

_Forceplate

_{The forceplate orientation and reference marker is used to locate the forceplate with respect to the calibrated reference frame. The APASview supports two different types of forceplates AMTI and Kistler. The forceplate setup and sampling must be done within the APAS analog module.}

_{Before the marker trajectory is digitised a reference marker to locate the forceplate in the global co-ordinate system must be selected.}

_{The configuration of the calibration frame determines the position of the reference marker. There is four different ways the calibration frame can be configured. The reference marker must be placed in the corner of the forceplate that resides closest to the calibration frame origin 0,0,0.}

_Example:

_{The orientation of the forceplate within the calibration frame is also an important parameter for calculating the correct COP and displaying the proper force vectors.}

_{Four different orientation can be selected in the APASview. On both pictures covering the forceplate orientation two markers are placed, indicating where the marker must be placed for the correct setup.}

Kistler orientation

AMTI orientation

_{The forceplate orientation and proper reference marker selection will enable the APASview to display and calculate the COP and force vectors in the exact the same way, despite the orientation of the forceplate with respect to the calibration frame.}

_{Important: The calibration frame must be exactly aligned with the forceplate.}

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