Considerations

Categories

Practical implementation

When planning the analysis of lower extremity kinetic and kinematics, some considerations about hardware setup must be carried out.

Basic considerations

When mounting markers on the subject it is important that a standardized procedure must be used. When palpating the bony landmarks, it is important for the person doing the task to be able to find the right position.

In order to detect markers on video the marker size can be crucial when auto digitizing with the 3DKin software. The best way of choosing the optimal reflective marker size is to experiment with various types and sizes.

It can be helpfull to have the subject wear thin black pants. This will enhance the contrast of the marker in the auto digitizing proces.

Camera

Position

When tracking markers in 3D using video, two or more cameras must be able to identify the markers. If both legs must be analyzed it is important to have enough cameras covering all 15 body markers. In some cases it is difficult with only two or three cameras to identify all markers perfectly, four or more cameras must therfore be considered. If only two or three cameras will be used the position of the cameras must be extremly carefully choosen. An optimal solution would be a five camera setup like the following example:

Speed

When filming human movement it is important to consider the speed of the cameras in respect to the actual speed of movement. The impact of camera speed can be crucial if movement is too fast for the camera. If the movement of interest is considerd fast, a high speed (100-200Hz) trial analysis can be done. Using this data the frequency components of the movement can be found and the optimum camera speed chosen. In gait analysis 50/60 HZ provided with video cameras or camcorders are normally considered enough.

Shutter

Normally the shutter speed would be chosen as the highest possible for the camera in use. This will in most cases be a shutter speed of 1/1000 of a second. Using this shutter speed a large amount of light must be provided and therefore is not always a good choice. If enough light can be provided it is recommended to use the highest speed possible. When planning the analysis of lower extremity kinetics and kinematics a video take sample should be performed and viewed on the APAS system to optimize shutter speed and light.

Synchronization

When using more than two cameras it is important to synchronize the cameras, so that the time of exposure is started at the same time for all cameras. This is nomally done using cameras with genlock systems. Genlock is a synchronization system where a single camera is master and all other cameras slave. The master camera provides all other cameras with a synchronization signal (the video signal). Using camcorders synchronization is not always possible. If not compensated for desyncronization the results of the analysis will be with errors.

Lights and background

Automatic digitizing is extremly sensitive to the background appearance. In the following example equipment and other "noise" is visible in the background.

If auto digitizing should be used on this example the 3DKin program would probably detect wrong coordinates for some of the markers.

To avoid problems in automatic digitizing remove all objects not used in the video take, and cover the background with black sheets. If it is possible, painting the room in dark colors and using dark carpets for the floor and walk way must be considered.

The lights in the room can also influence the quality of the video picture. If the light is exposed directly on the subject reflections can sometimes be seen on the video source. To avoid reflections the light can be directed to the ceiling and expose the subject to diffuse light. The choice of light type (bulb, neon or halogen) can also be crucial to the picture quality and therfore must be taken into consideration.

Forceplate

The 3Dkin program supports two types of forceplates, AMTI and Kistler.

When planning the walkway the position of the forceplate(s) can influence the pattern of movement. If fx. the initial approach to the forceplate is too short, the movement pattern of interest could be influenced, and also if the distance to next object (fx. wall) is to close to the forceplate the subject would probably try to avoid the object and the movement analyzed is not the one intended. Last the distance between the forceplates is also a parameter that can influence the movement pattern.

It is very important when planning the analysis to consider what kind of movement to be analyzed, and how it can be implemented in the laboratory. The optimal is that no influence is made on the subject but in real life the task will be to bring down the amount of parameters that can influence the subject.

Calibration cube/frame

The calibration cube/frame is the object used to transform marker trajectory in to the global coordinate system. In the literature many different kinds of calibration cubes/frames have been used, and it seems that the shape is not that important. The size of the calibration cube/frame can in contrast to the shape have impact on the accuracy. If a small calibration cube/frame is used a body or forceplate marker will be contaminated with small errors. The calibration cube/frame must have a size equal or larger to the size of the whole movement.

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