Reaction-Oriented Coordinate System
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Reaction-Oriented Coordinate System
Data Conversion

Reaction-Oriented Coordinate System

Commercial ground reaction force analysis packages such as the one for the AMTI plates use the action-oriented coordinate system (Figure 1a). In other words, the force exerted on the plate by the human body is to be measured and analyzed. This causes confusion when we use the reaction-oriented terms such as braking force and propulsion. Besides, we often combine the kinematic data obtained through motion analysis and the ground reaction force data to compute the joint torque and the net joint force using the inverse dynamics approach. The inverse dynamics procedures force investigators to view the ground reaction force as an external force acting on the body. This is why it is more advantageous to use the reaction-oriented coordinate system (Figure 1b). KwonGRF Ground Reaction Force Analysis Package consistently uses the reaction-oriented coordinate system with the origin at the center of the plate surface. This coordinate system is compatible to that used in Kwon3D.

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Data Conversion

One must be very cautious in converting GRF-related data from the action-oriented coordinate system to the reaction-oriented. For example, the action in +X direction in Figure 1a is equivalent to the reaction in the +X direction in Figure 1b. The action in the +Y direction in Figure 1a is equivalent to the reaction in the -Y direction in Figure 1b. The action in the +Z direction in Figure 1a is equivalent to the reaction in the +Z direction in Figure 1b.

In converting the data from the action-oriented system (both Kistler & AMTI) to the reaction-oriented system (KwonGRF), one must reverse the sign of the Y components of the GRF and the moment. One must also reverse the signs of the X & Z components of the position data such as the position of the true origin and the center of pressure (CP) position (see Center of Pressure for the details of the true origin and the CP):

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Young-Hoo Kwon, 1998-