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The <marker_file> property specifies a .trc file containing marker trajectories for this trial. These trajectories give the experimental marker positions which IK will attempt to match as described in __40__How Inverse Kinematics Works.

The <coordinate_file> property specifies a .mot file containing experimental coordinate values (e.g., joint angles computed using anatomical reference frames) for this trial. These values are read in from the file only if there is an IK Task for the coordinate that has a <from_file> tag set to true (see <IKCoordinateTask> in the Example IK Task File below).

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The <weight> property specifies the weight multiplying the squared-error term for that marker. These weights are the w_i's appearing in the formula given in __40__How Inverse Kinematics Works. A larger weight means that the error term for that marker will be more significant in the least-squares equation, so the IK computation will try to find a closer match between that marker's position and its experimental position, as compared to a marker with a smaller weight. By default, the weight for a marker is assumed to be zero, indicating that the IK solver will not attempt to match that marker to its experimental position. The default weight is used if <weight> is not specified or if the <IKMarkerTask> is omitted for a marker.

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The <weight> property specifies the weight multiplying the squared-error term for that coordinate. These weights are the ω_i's appearing in the formula given in __40__How Inverse Kinematics Works. They tell the IK solver how closely it should try to match a given coordinate to its experimental value: the larger the weight, the closer the match should be.

Recall from __40__How Inverse Kinematics Works that there are two types of coordinates: prescribed and unprescribed. If the OpenSim model specifies a coordinate to be locked (the <IKCoordinateTask> has <locked> set to true), then it is a prescribed coordinate. The lock symbol in the __40__Coordinates Window will also be in the locked position for this coordinate. Prescribed coordinates will be assigned an exact value and will not be solved for by IK. Thus, the <weight> property has no effect on prescribed coordinates; it is only relevant for unprescribed coordinates.

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