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3.0 MOTION ANALYSIS PROCESS &nbnsp;  A typical movement analysis consists of four distinct phases; Data Collection (Filming), Digitizing, Computation, and Presentation of Results. Data collection is the only phase that is not computerized, i.e. video performance does not require a computer because the camera is used independently from the APAS hardware. The organization of the major software modules in the APAS are illustrated in the flowchart in
[Figure 3.1].

3.1 DATA COLLECTION   In this phase, film or video recording of an activity are made using two or more cameras. The rules for data collection allow great flexibility in the recording of an activity. Information about the camera location and orientation, the distance from camera to subject, and the focal length of the lens is not required. The image space is calibrated through the use of calibration points that do not need to be present during the actual performance of the activity. Different types of cameras and different film speeds can be used and the cameras do not need to be mechanically or electronically synchronized. The optimal camera settings are obtained when camera viewing axes are orthogonal (90 degrees apart), but only variations which allow at least a 3D angle between the cameras can be accommodated while introducing almost negligible error.

3.2 DIGITIZING PHASE   Digitizing is the second phase of analysis and is primarily a manual process, although the APAS also supports an Automatic Digitizing Feature. Manual digitizing is performed under computer control and the digitizing of video images is computer assisted. Under manual control, user participation in the digitizing process provides an opportunity for error checking and visual feedback which rarely slows the digitizing process adversely. A trained operator with a reasonable knowledge of anatomy and a consistent pattern of digitizing can rapidly produce high-quality digitized images. Because all subsequent information is based on the data provided in this phase, it is essential that the points are selected precisely.

Automatic digitizing is available and requires high contrast visible markers, which the system can automatically trace after the first two frames have been manually digitized. Markers can be light or dark compared to the immediate background.

3.2.1 Grabbing Module   Initially, the video image is captured by the computer and stored in memory by use of the Grabbing Module. This eliminates the need for the video apparatus during digitizing. The image sequence is then retrieved from computer memory and is displayed, one frame at a time, on the digitizing monitor. The grabbed image can be enhanced or altered in several ways. These include zooming the whole frame or a defined, isolated portion of the view. Changing the size may help the person digitizing to more accurately determine a particular joint location. It is also possible to adjust the coloring, shading, and intensities of the image for each frame prior to grabbing. Thus alterations of the video image, in a manner similar to camera adjustments, can assist in the digitizing process.

3.2.2 Digitizing Module   Digitizing body joints can be performed from the stored (grabbed) video image or directly (live) from the video tape. The grabbing procedure is recommended when compared to live digitizing since mechanical variations of the VCR are controlled. Using a video cursor, the location of each of the subject's body joints (e.g. ankle, knee, hip, shoulder, elbow) is selected and entered into the computer. As each point is selected, it is displayed within the video image on the monitor.

3.3 COMPUTATION PHASE   The computation phase of analysis is performed after all camera views have been digitized. The purpose of this phase is to compute the true three-dimensional image space coordinates of the subject's body joints from the relative two-dimensional digitized coordinates of each camera's view.

3.3.1 Transformation Module   This module converts digitized film or video data into true two- or three-dimensional image data. Computation is performed using any of three possible transformation algorithms. Users may select Multiplier (2D), Direct Linear Transformation ( 2D or 3D DLT), and Physical Parameter Transformation (2D or 3D with Panning option). The transformation process consists of two distinct phases:

  1. Time synchronization of all simultaneous camera views used to record the activity (from one to nine cameras with varying frame rates may be used).
  2. Computation of the true image coordinates for the body joints in each frame from the multiple sets of digitized coordinates.

The Multiplier method is only recommended if the other two algorithms cannot be used. This method requires that the location of two points be known. All measurements are then based on the single known distance.

The DLT can be used for 2-Dimensional or 3-Dimensional analysis. Two dimensional analysis requires a minimum of 4 co-planar points (the Z coordinate must be zero for all points). Three dimensional analysis requires a minimum of 6 non co-planar points.

The PPT algorithm is required for views using the Ariel Panorama hardware, however, it can also be used with stationary camera views.

If two or more cameras are used, the resulting image is three-dimensional. If only a single camera is used, the result is two-dimensional.

3.3.2 Smoothing Module   When the transformation is complete, a smoothing or filtering operation is performed on the image coordinates to remove small random digitizing errors and to compute body joint velocities and accelerations. The user may choose between five different types of smoothing functions: cubic spline, digital filter, polynomial, quintic spline and Fourier smoothing. Smoothing may be performed automatically by the computer or interactively with the user controlling the amount of smoothing applied to each joint to ensure that smoothing does not distort the digitized data.

3.3.3 Kinetics Module   Body joint displacements, velocities and accelerations are combined with body segment mass distribution to compute dynamic forces and moments at each of the body joints. External forces can be added to the computation. Final analysis of energy, forces, momement and torque can be produced using the Graphing module.

3.4 PRESENTATION PHASE   The next 4 modules are used in presentation of the image motion data computed by the analysis process. The presentation phase of the analysis allows computed results to be viewed and recorded in a number of different formats. Three dimension body position and motion can be presented in both still frame and animated "stick figure" format. Multiple stick figures may be played simultaneously for comparison purposes. Joint velocity and acceleration vectors may be added to the stick figure to show the magnitude and direction of body motion parameters. Color hard copies of these displays can also be produced for reporting and publications. Any or all of them may be used in a typical analysis in any order or combination. The purpose of these modules is to allow examination of the results of an analysis in various forms, they do not change or transform the data in anyway.

3.4.1 Viewing Module   The Viewing Module is used to examine image data in stick figure format on the color graphic display. Stick figures are generated by connecting the body joints with lines corresponding to the body segments. Viewing options include single frame, multiple frame, and animated images. If three-dimensional images have been computed, the image may be rotated to allow viewing from any chosen direction. Images from as many as four separate sequences may be displayed at the same time for comparison purposes and image size and location may be changed in any desired way. Text labels can be added to the display to allow the generation of complete illustrations for use in publications and reports.

3.4.2 Graphing Module   The Graphing Module is used to draw displacement, velocity, acceleration, energy, momentum, or kinetic curve graphs for any number of individual body joints or segment. Body joint motion may be presented in either linear values (distance units) or in angular values (measured in degrees). Body segment motion is presented in angular coordinates about a single segment end-point. Stick figure images may be drawn to illustrate the body position at specific points along motion curves. Text labels may also be added to enhance the graphs. Tables of numeric data values corresponding to the graphed data may be printed or saved to worksheet or ASCII files.

3.4.3 Printing Module   The printing module is used to produce printed reports of the image motion data. Reports may be generated by individual joint or segment, or by individual motion parameters for all joints or segments. Reports may be viewed on the monochrome display before printing or in lieu of printing. In addition, reports may be saved in files for later printing or written to DIF or WKS files for transfer to other systems such as spreadsheets or database programs.

3.4.4 Analog Module   This is an optional module that performs generalized laboratory data measurement and analysis on as many as 32 simultaneous channels of analog input. Specialized features of this module support the use of biomechanics force platforms and motion sensors. This allows the direct measurement of human performance analysis. In addition, the analog module includes specialized support for electromyogram (EMG) measurement and analysis. EMG processing options (which may also be applied to other types of data) include spike analysis, envelope processing, signal integration, waveform analysis, and power spectrum analysis. The analog module includes a hardware analog interface and sampling unit. Analog measurements may also be saved in data files for later analysis or written to ACSII, DIF or WKS files for transfer to spreadsheet or database systems.

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