The error sensitivity in the determination of center and angle of rotations of a body joint performing planar motion is studied. A simple experiment is described to measure the errors in these two kinematic parameters as functions of errors in the input coordinates of markers. The effect of varying the marker locations and the size of motion-step is also studied. The errors in the center of rotation are found to increase dramatically when the two markers subtend angles of about 0 deg or 180 deg and when the motion step size decreases to 1 deg or less. Similar results are found for errors in the angle of rotation which, in addition, increase with decrease in the radius of the markers.